The irradiated GaAs sample was provided by Yang Cui of the University of Illinois at Chicago. This work was supported under Contract No. DE-AC02-06CH11357 between UChicago Argonne, LLC and the U.S. Department of Energy and by NASA through grants NNH08AH761 and NNH08ZDA001N, and the Office of Vehicle Technologies of the U.S. Department of Energy under contract DE-AC02-06CH11357. The electron microscopy was accomplished at the Electron Microscopy Center for Materials Research at Argonne National Laboratory, a U.S. Department of Energy Office of Science laboratory, operated under Contract DE-AC02-06CH11357 by UChicago Argonne, LLC.

1. Hardware Alignment for General WLI Scan
To obtain quantitative information through WLI, the following steps may serve as a guideline. It is assumed that the operator has basic knowledge of interferometer operation. The guidelines are common regardless of the specific instrument. For some investigations, the specimen will be flat. For others, the specimen may be curved.
<ol>
	<li>Place the sample on the stage with the feature (ion sputtered crater, ion beam/ablated spot, or wear scar) facing straight up. Use a low-magnification objective and focus the instrument on it. For best resolution obtain an image in which the object of interest largely fills the screen, see Figure 2 for an example of a ball.</li>
	<li>Adjust the vertical position of the specimen so that interference fringes appear near the feature of interest. For a flat surface it is desirable that the specimen is tilted such that the plane is perpendicular to the optical axis, i.e. the fringe spacing will be large. For a curved surface (e.g. a ball), the specimen should be oriented such that fringes are centrally located around the feature, such as in Figure 3.</li>
	<li>Obtain a scan according to the instrument instructions. It may be necessary to adjust the illumination or scan height in order to obtain the best topographic map. Fill in any bad or missing data using the interpolate function, and then save the topographic map.</li>
</ol>
2. Volume Analysis Using Generic Software
In tribology, testing machines often employ non-conformal contact testing geometries such as a ball or a cylinder that is slid or rotated against a flat or another cylinder. Typically, material is lost at the point of sliding contact, although sometimes material may transfer from one surface to another, and this "transfer layer" will be measured as a surplus of material on one of the contacting specimens. Because the wear scar is three-dimensional and irregular in nature, optical profilometry may be the only technique suitable for obtaining accurate wear volume measurements - approximations are not valid. The goal is to measure the very small amounts of material which may be lost (or gained) in the contacting region at the completion of a test.
The basic principle of measurement is to define a mathematical plane at the level of the undisturbed surface: surface analysis software assumes that there is a "solid surface" (zero level), whatever above this level is "empty." The analysis function that measures the integrated removed volume below the plane of the "solid surface" will be referred to as "void volume." The function that measures the integrated volume above the plane of the "solid surface" (e.g. an accumulation of debris) will be called the "material volume."
Real undisturbed surfaces are seldom perfectly smooth and flat. For greatest accuracy in measuring small features it is good practice to define an area of interest (AOI); the area outside of which is excluded from analysis. The AOI is used to restrict the measurement area because irregularities in the surface contribute additional volume to the calculations that is not truly from the disturbed area.
 
3. Flat Surface - Mechanical Wear Analysis
<ol>
	<li>Volume analysis of a flat surface with a trench scar or depression, whether generated by mechanical wear, ion beam sputtering, or laser ablation, is easy to perform. Obtain an image such as Figure 4 left which shows a mechanical scar on a polished steel surface. Select an AOI that excludes the depression and then use whatever tilt and/or curvature removal function is available to level the undisturbed surface to maximum flatness. Then use the software to set the average surface height to Z=0. Figure 4 right is a pseudocolor view of a scar on a flat surface after these steps. In this view the excluded "void volume" is tinted red. In this color scheme, the dark brown denotes low areas, while orange denotes high areas.</li>
	<li>Delete the AOI. If the software has an automated volume measuring function, place a region of measurement over the scar. In this software, the measuring function will tally the "void and material volume" shown with blue tint in Figure 5. Total wear is "material volume" above the plane subtracted from "void volume." This is the volume change of the disturbed surface. (For software that does not have this function, but does have a histogram, or to clearly visualize which areas are raised above the surface or are below the surface, skip step 3.2 and go to step 3.3.)</li>
</ol>
The following three steps describe an alternate method of measuring wear volume.
<ol start="3">
	<li>Invert the AOI from step 3.1 to enable the scar region to be analyzed (the scar is included now).</li>
	<li>Generate a histogram of the data. The histogram is a graph of vertical height on the abscissa vs. the frequency of occurrence on the ordinate. Place one cursor at the Z=0 position (Figure 6 left). This may not be at the peak of the histogram. In this figure the cursors have been set up to selectively analyze only the missing material below the plane. The software integrates the total volume between the two cursors. The red tinted area shows what is omitted. The orange untinted areas in Figure 6 right are below the surface. The histogram function should generate a "void volume" number with the cursors positioned as shown to measure the material lost from the surface.</li>
	<li>Using the same histogram, place the other cursor this time at the Z=0 height position (Figure 7 left), and the other at the opposite end. The orange untinted areas in Figure 7 right are above the surface. The histogram function should generate a "material volume" number, i.e. surplus material raised above the flat surface. Total wear is "material volume" subtracted from "void volume," the same as in step 3.2. The histogram method should give the same wear volume as in step 3.2, but it provides additional details about the distribution of raised and lowered material, and shows a map of the distribution of this area.</li>
</ol>
In the example above there is no net material lost from the wear scar, instead there is material gain. This is unusual, but sometimes occurs when material transfers from one test counterface to another.
The same "flat surface characterization" approaches are beneficial for obtaining volumes removed in experiments with ion sputtering and laser ablation, considered in examples that follow.
4. Flat Surface - Craters and Ion Beam Profiles Measurements to Estimate Sputtering Yield, and to Perform Time-to-depth Calibration
As an alternative to the known and widely employed method for estimating sputtering yields using mass-loss method, based on direct weighing or quartz microcrystal balance,8, 9 we find that the WLI method is useful for direct visualization of the sputtered ion beam spots/craters obtained by static sputtering/raster scanning of an ion beam. Figure 8 compares longitudinal cross sections of a spot (green solid and blue dotted lines) of a normally incident static 5 keV and 150 eV Ar+ ion beams against their craters (olive open circles and cyan open diamonds) obtained by 100&times;100 pixels digital raster scanning of the same ion beam over the surface of a Cu(110) single crystal. The curve corresponding to the static beam overlaps one edge of the crater to demonstrate how raster scan of the ion beam generates the crater during sputter depth profiling.
5. Flat Surface - Characterization of Ultrafast Laser Ablation
Ultrafast laser ablation is recognized as a method for removing material from a solid while minimizing the heat affected zone.10 This process enables high rate micromachining with high aspect ratios and minimal damage (cracking, oxidation) to the remaining material, and opens up the possibility of efficient ablation of transparent materials.11 More recently, interest has developed in using ultrafast ablation as an analytical tool.12-15 The high nonlinearity of the ablation process also provides a means of reducing the ablated spot size significantly below the irradiated spot size (as typically defined by 1/e2) and even below the diffraction limit, as has been demonstrated.16 Depth resolution, while not competitive with the best ion beam methods, may be &lt;20 nm. Removal rates can easily be increased nonlinearly by increasing the laser fluence, so that very rapid profiling through microns of material is possible. Ideally, characterizing material removal by ultrafast ablation requires a technique that is fast and quantitative and well calibrated, all characteristics fulfilled by WLI.
Figure 9 shows pseudocolor images of two neighboring craters formed by repetitive ablation of GaAs with an ultrafast (60 fs, 800 nm) laser beam focused to a spot size of &asymp;8 &mu;m and having fluences corresponding to 0.4 and 1.0 J/cm2.
6. Curved Surfaces - Mechanical Wear Analysis
Volume analysis of a curved regular surface (ball or cylinder) is similar to that of a flat, but requires curvature removal. The following protocol shows an analysis of a circular wear scar on a steel ball. To find the volume lost from a ball it is necessary to do mathematical processing to transform a ball with a flat area into a flat plane with an indentation, then measure the volume of the indentation as was done in Section 3 on flat surfaces. A wear scar on a ball will be measured, first using the simpler automatic technique, then with the histogram technique.
<ol>
	<li>Figure 10 left shows an isometric view of a wear scar on a ball. Select an AOI that excludes the wear scar, and select the software curve fitting tool that will transform the surface so that it is a worn depression in the middle of an undisturbed flat area. Because curvature removal may be an iterative technique it may be necessary to run the fit several times so that the undisturbed area is flat to nm level accuracy. Any visible non-uniformity outside of the wear scar indicates a problem and the calculation will not be correct. Set the average height outside the scar to Z=0. Figure 10 right shows a pseudocolor view of the scar after curvature removal and setting Z=0 with an AOI properly masking the worn area.</li>
	<li>Use the measuring tool, if available, to analyze the wear, as shown in Figure 11. Total wear volume is "material volume" subtracted from the "void volume."</li>
</ol>
The following steps describe an alternate method of measuring wear volume.
<ol start="3">
	<li>In the same manner as step 3.3, invert the AOI so that the wear scar is included. Generate a histogram of the data. Place one cursor at the Z=0 position (Figure 12 left). The orange untinted areas in Figure 12 right are below the surface. The histogram function should generate a "void volume" number.</li>
	<li>Using the same histogram, place the other cursor this time at the Z=0 height position (Figure 13 left), and the other at the opposite end. The orange untinted areas in Figure 13 right are above the surface. The histogram function should generate a "material volume" number. Total wear volume is "material volume" above the plane subtracted from "void volume," the same as in step 3.2. The histogram method should calculate the same wear volume as in step 6.2, but it provides additional details about the distribution of raised and lowered material, and shows a map of the distribution of this area.</li>
</ol>

No conflicts of interest declared.

Example 1
WLI is not widely used for surface characterization in tribological work, but it is in fact a powerful method for quantitative measurement of wear volumes for many contact geometries. WLI produces a full 3D representation of the surface that can be analyzed using any of several visualization software packages. These packages enable various types of measurements to be performed. For greater lateral resolution, images can be "stitched" together to produce wide-area information (several mm...

The surface of solid materials determines to a large extent properties of interest for those materials: electronically, structurally, and chemically. In many areas of research, the addition of material (for instance, thin film deposition by pulsed laser/magnetron sputtering deposition, physical/chemical vapor deposition), removal of material (reactive ion etching, ion sputtering, laser ablation, etc.), or some other processes, need to be characterized. Additionally, surface modification through interaction with energetic light pulses or charged particles has numerous applications and is of fundamental interest. Tribology, the study of friction and wear, is another area of interest. On a benchtop scale, a multitude of tribological test geometries exist. Non-conformal contact geometries may be used, and a ball or cylinder may be slid or rotated against a flat surface, another ball, or cylinder, for a length of time, and the amount of material that is removed is measured. Because the wear scar is three-dimensional and irregular in nature, optical profilometry may be the only technique suitable for obtaining accurate wear volume measurements. Common analysis tasks include also surface roughness parameters, step height, loss of material volume, trench depth, and so on; all of them can be obtained additionally to simple 2D and 3D topography visualization.
Optical profilometry refers to any optical method that is used to reconstruct the profile of surfaces. Profilometric methods include white light interferometric, laser, or confocal methods. Some optical profilometers obtain information through approaches based on conventional diffraction-limited microscope objectives. For example, a scanning laser may be integrated with a microscope to obtain topographic and true color information of surfaces. A second method uses a technique which exploits the extremely small depth of focus of conventional objectives to assemble a series of in-focus "image slices" of the surface to obtain a 3D topographic map.
In this work we show how a white light interferometric microscope/profilometer enables the measurement of the amount of material lost during mechanical wear processes, or during material etching processes such as ion sputtering craters or laser ablation. Most attention is paid to methodology of this method to illustrate its large installed capacity that makes it widely available and attractive for numerous applications. Most types of WLI employ the Mirau technique, which uses a mirror internal to the microscope objective to cause interference between a reference light signal and the light reflected from the sample surface. The choice of Mirau interferometry is dictated by simple convenience, because the entire Mirau interferometer can be fit inside the microscope objective lens and coupled to a regular optical microscope (Figure 1). A series of two-dimensional interferograms are acquired with a video camera, and software assembles a 3D topographic map. The white light source supplies broad spectrum illumination which helps to overcome the "fringe order" ambiguity inherent to a monochromatic source. A monochromatic source of light may be used to obtain more accurate measurement of shallow topographic features. The lateral resolution is fundamentally limited to &lambda;/2 (numerical aperture, NA=1), but in most instances is larger, being determined by the NA of the objective, which is in turn connected to magnification/field-of-view size. Table 1 in Ref. 1 has a direct comparison of all mentioned parameters. Depth resolution approaches &asymp;1 nm, being a function of the interferometric nature of the technique. Further information on Mirau WLI can be found in Refs. 2, 3. An introduction on white light interferometric approach can be found in Ref. 4.
Other methods for analysis of surfaces are atomic force microscopy (AFM), scanning electron microscopy (SEM), and stylus profilometry. The WLI technique compares favorably to these methods and has its own advantages and drawbacks that are due to the optical nature of the method.
The AFM is capable of obtaining 3D images and thus corresponding cross sections, but AFM has a limited scanning ability in the lateral (&lt;100 &mu;m) and depth (&lt;10 &mu;m) axes. In contrast to those, the main advantage of WLI is the flexible field-of-view (FOV) of up to a few millimeters with simultaneous real 3D imaging capability. In addition, as we will demonstrate it has wide vertical scanning range capacity, allowing one to solve a variety of problems of surface modification simply. Researchers who have worked with AFM are aware of the problem with plane positioning of a sample when measuring prolonged features of low vertical gradients. Generally, one may think of WLI/OP as an "express" technique over AFM. Of course, there are a number of areas for which only AFM is suitable: when lateral features to be resolved have characteristic dimensions smaller than the lateral resolution of WLI, or instances where data from WLI is ambiguous due to unknown or complex optical properties of a sample in a way that affects the accuracy of measurements (to be discussed later), etc.
The SEM is a powerful way to look at surfaces, being very flexible in terms of the FOV size with large depth of focus, larger than any conventional optical microscope can offer. At the same time, 3D imaging by SEM is cumbersome, particularly as it requires taking of stereo-pair images that then are converted to 3D images by the anaglyphic method, or through observing with optical viewers, or used for direct calculation of depths between different points of interest on a sample.5 By contrast, WLI/OP profilometry offers easy-to-use 3D reconstruction with simultaneously flexible FOV. WLI scans through the full height range needed for the particular sample (from nanometers to hundreds of microns). WLI is unaffected by the electrical conductivity of the sample material, which may be a problem with SEM. WLI clearly does not require a vacuum. On the other hand there are a number of applications for which SEM provides superior information: lateral features to be resolved of characteristic dimensions below the lateral resolution of WLI, or cases where different parts of a sample can be topographically distinguished only when secondary electron emission coefficients differ.
One more technique for surface inspection, which is widely used in secondary ion mass spectrometry6 and in the field of microelectromechanical systems characterization7 is stylus profilometry. This technique is popular because of its simplicity and robustness. It is based on direct mechanical contact scanning of a stylus tip over the sample surface. This is a coarse contact tool, which is able to scan along a single line at a time. It makes 3D surface raster-scan imaging extremely time consuming. Another drawback of the stylus technique is the difficulty of measuring surface features of high aspect ratio and of size comparable with its characteristic tip size (submicron to several microns typically) that implies a tip radius and a tip apex angle. An advantage of stylus profilometry is its insensitivity to varying optical properties of a sample, which can affect the accuracy of WLI/OP measurements (to be discussed later).
The surface maps in the present article were obtained using a conventional Mirau-type WLI (Figure 1). Many companies such as Zygo, KLA-Tencor, nanoScience, Zemetrics, Nanovea, FRT, Keyence, Bruker, and Taylor Hobson produce commercial table-top OP instruments. The acquired maps were reconstructed and processed using commercial software of the type that is commonly used for WLI, scanning electron, or probe microscopy. The software has the ability to perform mathematical manipulations of the surface, cross section profile analysis, void and material volume calculations, and plane correction. Other software packages may automate some of these features.

<table><tbody><tr><td>Single crystal substrates of Si, GaAs and Cu </td><td></td><td></td><td>for sputtering and ablation </td></tr><tr><td>Pure metal alloys </td><td></td><td></td><td>for tribology examples</td></tr></tbody></table>

characterization of surface modifications by white light interferometry: applications in ion sputtering, laser ablation, and tribology experiments

In materials science and engineering it is often necessary to obtain quantitative measurements of surface topography with micrometer lateral resolution. From the measured surface, 3D topographic maps can be subsequently analyzed using a variety of software packages to extract the information that is needed.
In this article we describe how white light interferometry, and optical profilometry (OP) in general, combined with generic surface analysis software, can be used for materials science and engineering tasks. In this article, a number of applications of white light interferometry for investigation of surface modifications in mass spectrometry, and wear phenomena in tribology and lubrication are demonstrated. We characterize the products of the interaction of semiconductors and metals with energetic ions (sputtering), and laser irradiation (ablation), as well as ex situ measurements of wear of tribological test specimens.
Specifically, we will discuss:
<ol class="lroman">
	<li>Aspects of traditional ion sputtering-based mass spectrometry such as sputtering rates/yields measurements on Si and Cu and subsequent time-to-depth conversion.</li>
	<li>Results of quantitative characterization of the interaction of femtosecond laser irradiation with a semiconductor surface. These results are important for applications such as ablation mass spectrometry, where the quantities of evaporated material can be studied and controlled via pulse duration and energy per pulse. Thus, by determining the crater geometry one can define depth and lateral resolution versus experimental setup conditions.</li>
	<li>Measurements of surface roughness parameters in two dimensions, and quantitative measurements of the surface wear that occur as a result of friction and wear tests.</li>
</ol>
Some inherent drawbacks, possible artifacts, and uncertainty assessments of the white light interferometry approach will be discussed and explained.

<img alt="Figure 1" src="/files/ftp_upload/50260/50260fig1.jpg" /> 
 Figure 1. Photograph of a simple profilometer used in the present study: a multiple objective turret is seen in the picture. Two objectives are standard (10x and 50x), and two are Mirau objectives (10x and 50x). This microscope has an intermediate magnification feature that enables step-wise magnification multipliers of 0.62, 1.00, 1.25, or 2.00 to be selected. <a href="https://www.jove.com/fi...

Watch this Scientific Journal Video about Characterization of Surface Modifications by White Light Interferometry: Applications in Ion Sputtering, Laser Ablation, and Tribology Experiments at JoVE.com

Characterization of Surface Modifications by White Light Interferometry: Applications in Ion Sputtering, Laser Ablation, and Tribology Experiments

White light microscope interferometry is an optical, noncontact and quick method for measuring the topography of surfaces. It is shown how the method can be applied toward mechanical wear analysis, where wear scars on tribological test samples are analyzed; and in materials science to determine ion beam sputtering or laser ablation volumes and depths.

In materials science and engineering it is often necessary to obtain quantitative measurements of surface topography with micrometer lateral resolution. ...

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15.5K Views.  Argonne National Laboratory. White light microscope interferometry is an optical, noncontact and quick method for measuring the topography of surfaces. It is shown how the method can be applied toward mechanical wear analysis, where wear scars on tribological test samples are analyzed; and in materials science to determine ion beam sputtering or laser ablation volumes and depths.

Video: Characterization of Surface Modifications by White Light Interferometry: Applications in Ion Sputtering, Laser Ablation, and Tribology Experiments

In Situ SIMS and IR Spectroscopy of Well-defined Surfaces Prepared by Soft Landing of Mass-selected Ions

Soft landing of mass-selected ions onto surfaces is a powerful approach for the highly-controlled preparation of materials that are inaccessible using conventional synthesis techniques. Coupling soft landing with in situ characterization using secondary ion mass spectrometry (SIMS) and infrared reflection absorption spectroscopy (IRRAS) enables analysis of well-defined surfaces under clean vacuum conditions. The capabilities of three soft-landing instruments constructed in our laboratory are illustrated for the representative system of surface-bound organometallics prepared by soft landing of mass-selected ruthenium tris(bipyridine) dications, [Ru(bpy)3]2+ (bpy = bipyridine), onto carboxylic acid terminated self-assembled monolayer surfaces on gold (COOH-SAMs). In situ time-of-flight (TOF)-SIMS provides insight into the reactivity of the soft-landed ions. In addition, the kinetics of charge reduction, neutralization and desorption occurring on the COOH-SAM both during and after ion soft landing are studied using in situ Fourier transform ion cyclotron resonance (FT-ICR)-SIMS measurements. In situ IRRAS experiments provide insight into how the structure of organic ligands surrounding metal centers is perturbed through immobilization of organometallic ions on COOH-SAM surfaces by soft landing. Collectively, the three instruments provide complementary information about the chemical composition, reactivity and structure of well-defined species supported on surfaces.

In Situ SIMS and IR Spectroscopy of Well-defined Surfaces Prepared by Soft Landing of Mass-selected Ions

Soft landing of mass-selected ions onto surfaces is a powerful approach for the highly-controlled preparation of novel materials. Coupled with analysis by in situ secondary ion mass spectrometry (SIMS) and infrared reflection absorption spectroscopy (IRRAS), soft landing provides unprecedented insights into the interactions of well-defined species with surfaces.

Soft landing of mass-selected ions onto surfaces is a powerful approach for the highly-controlled preparation of materials that are inaccessible using ...

Chemistry

Investigating Single Molecule Adhesion by Atomic Force Spectroscopy

Atomic force spectroscopy is an ideal tool to study molecules at surfaces and interfaces. An experimental protocol to couple a large variety of single molecules covalently onto an AFM tip is presented. At the same time the AFM tip is passivated to prevent unspecific interactions between the tip and the substrate, which is a prerequisite to study single molecules attached to the AFM tip. Analyses to determine the adhesion force, the adhesion length, and the free energy of these molecules on solid surfaces and bio-interfaces are shortly presented and external references for further reading are provided. Example molecules are the poly(amino acid) polytyrosine, the graft polymer PI-g-PS and the phospholipid POPE (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine). These molecules are desorbed from different surfaces like CH3-SAMs, hydrogen terminated diamond and supported lipid bilayers under various solvent conditions. Finally, the advantages of force spectroscopic single molecule experiments are discussed including means to decide if truly a single molecule has been studied in the experiment.

A protocol to couple a large variety of single molecules covalently onto an AFM tip is presented. Procedures and examples to determine the adhesion force and free energy of these molecules on solid supports and bio-interfaces are provided.

Atomic force spectroscopy is an ideal tool to study molecules at surfaces and interfaces. An experimental protocol to couple a large variety of single ...

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H2O2/H2O solution did not show measurable change in their initial contact angle (CA) ~75&#176;. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H2O2/H2O solution at 250 and 65 mJ/cm2, respectively, has decreased the CA to near 15&#176;, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafer&#8217;s surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of&#160;the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH3OH) using ArF laser operating at pulse fluence of 65 mJ/cm2 and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103&#176;. The XPS results indicate ArF laser induced formation of Si&#8211;(OCH3)x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH3 radicals.

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with&#160;H2O2/H2O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of ...

Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution for future high-speed reentry missions. But despite the advancements made since Apollo, heat flux prediction remains an imperfect science and engineers resort to safety factors to determine the TPS thickness. This goes at the expense of embarked payload, hampering, for example, sample return missions. 
Ground testing in plasma wind-tunnels is currently the only affordable possibility for both material qualification and validation of material response codes. The subsonic 1.2MW Inductively Coupled Plasmatron facility at the von Karman Institute for Fluid Dynamics is able to reproduce a wide range of reentry environments. This protocol describes a procedure for the study of the gas/surface interaction on ablative materials in high enthalpy flows and presents sample results of a non-pyrolyzing, ablating carbon fiber precursor. With this publication, the authors envisage the definition of a standard procedure, facilitating comparison with other laboratories and contributing to ongoing efforts to improve heat shield reliability and reduce design uncertainties.
The described core techniques are non-intrusive methods to track the material recession with a high-speed camera along with the chemistry in the reactive boundary layer, probed by emission spectroscopy. Although optical emission spectroscopy is limited to line-of-sight measurements and is further constrained to electronically excited atoms and molecules, its simplicity and broad applicability still make it the technique of choice for analysis of the reactive boundary layer. Recession of the ablating sample further requires that the distance of the measurement location with respect to the surface is known at all times during the experiment. Calibration of the optical system of the applied three spectrometers allowed quantitative comparison. At the fiber scale, results from a post-test microscopy analysis are presented.

Development of new ablative materials and their numerical modeling requires extensive experimental investigation. This protocol describes procedures for material response characterization in plasma flows with the core techniques being non-intrusive methods to track the material recession along with the chemistry in the reactive boundary layer by emission spectroscopy.

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution ...

The Evolution of Silica Nanoparticle-polyester Coatings on Surfaces Exposed to Sunlight

Corrosion of metallic surfaces is prevalent in the environment and is of great concern in many areas, including the military, transport, aviation, building and food industries, amongst others. Polyester and coatings containing both polyester and silica nanoparticles (SiO2NPs) have been widely used to protect steel substrata from corrosion. In this study, we utilized X-ray photoelectron spectroscopy, attenuated total reflection infrared micro-spectroscopy, water contact angle measurements, optical profiling and atomic force microscopy to provide an insight into how exposure to sunlight can cause changes in the micro- and nanoscale integrity of the coatings. No significant change in surface micro-topography was detected using optical profilometry, however, statistically significant nanoscale changes to the surface were detected using atomic force microscopy. Analysis of the X-ray photoelectron spectroscopy and attenuated total reflection infrared micro-spectroscopy data revealed that degradation of the ester groups had occurred through exposure to ultraviolet light to form COO&#903;, -H2C&#903;, -O&#903;, -CO&#903; radicals. During the degradation process, CO and CO2 were also produced.

Two types of surfaces, polyester-coated steel and polyester coated with a layer of silica nanoparticles, were studied. Both surfaces were exposed to sunlight, which was found to cause substantial changes in the chemistry and nanoscale topography of the surface.

Corrosion of metallic surfaces is prevalent in the environment and is of great concern in many areas, including the military, transport, aviation, ...

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

In liquid crystal (LC) physical chemistry, molecules near the surface play a great role in controlling bulk orientation. Thus far, mainly to achieve desired molecular orientation states in LC displays, the "static" surface property of LCs, so-called surface anchoring, has been intensively studied. As a rule of thumb, once the initial orientation of LCs is "locked" by specific surface treatments, such as rubbing or treatment with a specific alignment layer, it hardly changes with temperature. Here, we present a system exhibiting an orientational transition upon temperature variation, which conflicts with the consensus. Right on the transition, the bulk LC molecules experience the orientational rotation, with 90&#176; between the planar (P) orientation at high temperatures and the vertical (V) orientation at low temperatures in the first-order transitional manner. We have tracked thermodynamic surface anchoring behavior by means of polarizing optical microscopy (POM), dielectric spectroscopy (DS), high-resolution differential scanning calorimetry (HR-DSC), and grazing incidence X-ray diffraction (GI-XRD) and reached a plausible physical explanation: that the transition is triggered by a growth of surface wetting sheets, which impose the V orientation locally against the P orientation in the bulk. This landscape would provide a general link explaining how the equilibrium bulk orientation is affected by surface-localized orientation in many LC systems. In our characterization, POM and DS are advantageous by offering information on the spatial distribution of the orientation of LC molecules. HR-DSC gives information about the precise thermodynamic information on transitions, which cannot be addressed by conventional DSC instruments due to limited resolution. GI-XRD provides information on surface-specific molecular orientation and short-range orderings. The goal of this paper is to present a protocol for preparing a sample that exhibits the transition and to demonstrate how the thermodynamic structural variation, both in the bulk and on surfaces, can be analyzed through the abovementioned methods.

Here, we present a protocol to trigger an orientational transition of a liquid crystal in response to temperature. Methodologies are described for preparing a sample in order to observe the transition and the detailed transitional evolution.

In liquid crystal (LC) physical chemistry, molecules near the surface play a great role in controlling bulk orientation. Thus far, mainly to achieve ...

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. Although electrostatic mechanisms have been proposed in the past to describe the existence of this critical thickness, the importance of chemical defects has been recently accentuated. Here, we describe the growth of metal/LAO/STO heterostructures in an ultra-high vacuum (UHV) cluster system combining pulsed laser deposition (to grow the LAO), magnetron sputtering (to grow the metal) and X-ray photoelectron spectroscopy (XPS). We study step by step the formation and evolution of the q2DES and the chemical interactions that occur between the metal and the LAO/STO. Additionally, magnetotransport experiments elucidate on the transport and electronic properties of the q2DES. This systematic work not only demonstrates a way to study the electrostatic and chemical interplay between the q2DES and its environment, but also unlocks the possibility to couple multifunctional capping layers with the rich physics observed in two-dimensional electron systems, allowing the fabrication of new types of devices.

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures

We fabricate metal/LaAlO3/SrTiO3 heterostructures using a combination of pulsed laser deposition and in situ magnetron sputtering. Through magnetotransport and in situ X-ray photoelectron spectroscopy experiments, we investigate the interplay between electrostatic and chemical phenomena of the quasi two-dimensional electron gas formed in this system.

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide ...

Imaging Corrosion at the Metal-Paint Interface Using Time-of-Flight Secondary Ion Mass Spectrometry

Corrosion developed at the paint and aluminum (Al) metal-paint interface of an aluminum alloy is analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS), illustrating that SIMS is a suitable technique to study the chemical distribution at a metal-paint interface. The painted Al alloy coupons are immersed in a salt solution or exposed to air only. SIMS provides chemical mapping and 2D molecular imaging of the interface, allowing direct visualization of the morphology of the corrosion products formed at the metal-paint interface and mapping of the chemical after corrosion occurs. The experimental procedure of this method is presented to provide technical details to facilitate similar research and highlight pitfalls that may be encountered during such experiments.

Time-of-flight secondary ion mass spectrometry is applied to demonstrate the chemical mapping and corrosion morphology at the metal-paint interface of an aluminum alloy after being exposed to a salt solution compared with a specimen exposed to air.

Corrosion developed at the paint and aluminum (Al) metal-paint interface of an aluminum alloy is analyzed using time-of-flight secondary ion mass ...

Nanoscale Characterization of Liquid-Solid Interfaces by Coupling Cryo-Focused Ion Beam Milling with Scanning Electron Microscopy and Spectroscopy

Physical and chemical processes at solid-liquid interfaces play a crucial role in many natural and technological phenomena, including catalysis, solar energy and fuel generation, and electrochemical energy storage. Nanoscale characterization of such interfaces has recently been achieved using cryogenic electron microscopy, thereby providing a new path to advancing our fundamental understanding of interface processes.
This contribution provides a practical guide to mapping the structure and chemistry of solid-liquid interfaces in materials and devices using an integrated cryogenic electron microscopy approach. In this approach, we pair cryogenic sample preparation which allows stabilization of solid-liquid interfaces with cryogenic focused ion beam (cryo-FIB) milling to create cross-sections through these complex buried structures. Cryogenic scanning electron microscopy (cryo-SEM) techniques performed in a dual-beam FIB/SEM enable direct imaging as well as chemical mapping at the nanoscale. We discuss practical challenges, strategies to overcome them, as well as protocols for obtaining optimal results. While we focus in our discussion on interfaces in energy storage devices, the methods outlined are broadly applicable to a range of fields where solid-liquid interface play a key role.

Cryogenic Focused Ion Beam (FIB) and Scanning Electron Microscopy (SEM) techniques can provide key insights into the chemistry and morphology of intact solid-liquid interfaces. Methods for preparing high quality Energy Dispersive X-ray (EDX) spectroscopic maps of such interfaces are detailed, with a focus on energy storage devices.

Physical and chemical processes at solid-liquid interfaces play a crucial role in many natural and technological phenomena, including catalysis, solar ...

Preparation of Nanoparticles for ToF-SIMS and XPS Analysis

Nanoparticles have gained increasing attention in recent years due to their potential and application in different fields including medicine, cosmetics, chemistry, and their potential to enable advanced materials. To effectively understand and regulate the physico-chemical properties and potential adverse effects of nanoparticles, validated measurement procedures for the various properties of nanoparticles need to be developed. While procedures for measuring nanoparticle size and size distribution are already established, standardized methods for analysis of their surface chemistry are not yet in place, although the influence of the surface chemistry on nanoparticle properties is undisputed. In particular, storage and preparation of nanoparticles for surface analysis strongly influences the analytical results from various methods, and in order to obtain consistent results, sample preparation must be both optimized and standardized. In this contribution, we present, in detail, some standard procedures for preparing nanoparticles for surface analytics. In principle, nanoparticles can be deposited on a suitable substrate from suspension or as a powder. Silicon (Si) wafers are commonly used as substrate, however, their cleaning is critical to the process. For sample preparation from suspension, we will discuss drop-casting and spin-coating, where not only the cleanliness of the substrate and purity of the suspension but also its concentration play important roles for the success of the preparation methodology. For nanoparticles with sensitive ligand shells or coatings, deposition as powders is more suitable, although this method requires particular care in fixing the sample.

A number of different procedures for preparing nanoparticles for surface analysis are presented (drop casting, spin coating, deposition from powders, and cryofixation). We discuss the challenges, opportunities, and possible applications of each method, particularly regarding the changes in the surface properties caused by the different preparation methods.

Nanoparticles have gained increasing attention in recent years due to their potential and application in different fields including medicine, cosmetics, ...

Characterization of Surface Modifications by White Light Interferometry: Applications in Ion Sputtering, Laser Ablation, and Tribology Experiments

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Growth and Electrostatic/chemical Properties of Metal/LaAlO₃/SrTiO₃ Heterostructures

Imaging Corrosion at the Metal-Paint Interface Using Time-of-Flight Secondary Ion Mass Spectrometry

Nanoscale Characterization of Liquid-Solid Interfaces by Coupling Cryo-Focused Ion Beam Milling with Scanning Electron Microscopy and Spectroscopy

Preparation of Nanoparticles for ToF-SIMS and XPS Analysis