Application of Laser-Induced Breakdown Spectroscopy for Depth Profiling of Multilayer and Graded Materials

Laser-induced breakdown spectroscopy (LIBS) has emerged as a powerful analytical method for the elemental mapping and depth profiling of many materials. This review offers insight into the contemporary applications of LIBS for the depth profiling of materials whose elemental composition changes either abruptly (multilayered materials) or continuously (functionally graded or corroded materials). The spectrum of materials is discussed, spanning from laboratory-synthesized model materials to real-world products including materials for fusion reactors, photovoltaic cells, ceramic and galvanic coatings, lithium batteries, historical and archaeological artifacts, and polymeric materials. The nuances of ablation conditions and the resulting crater morphologies, which are instrumental in depth-related studies, are discussed in detail. The challenges of calibration and quantitative profiling using LIBS are also addressed. Finally, the possible directions of the evolution of LIBS applications are commented on.


Introduction
Today's advanced materials have evolved beyond simple single-component systems, embracing intricate multi-elemental compositions to achieve unprecedented properties.The pursuit of materials that can address the diverse and rigorous demands of contemporary applications has urged the development of a new generation of materials.These are not the monolithic single component materials of the past, as can be easily noticed by tracking how materials used in industries such as aviation have evolved over the years [1].Instead, the focus has been on complex multicomponent systems that amalgamate the best attributes of their components [2].Three categories stand out due to their innovative nature and adaptability: composites [3], multilayer materials [4], and functionally graded materials (FGMs) [5,6].While all three categories benefit from the advantages of blending different materials, they differ in structural organization and how these materials come together (Scheme 1).The primary distinguishing feature among them is the method in which the components are merged and distributed: (i) in composites, distinct phases are combined yet remain separate, ensuring even distribution within the material (Scheme 1a); (ii) multilayered materials consist of distinct layers stacked sequentially (Scheme 1b); and (iii) in FGMs, a smooth and continuous transition in composition and/or properties is observed across the material (Scheme 1c).Some examples of multilayer and diffusion-controlled materials are presented in Figures 1 and 2.
As the need for materials with customized properties grows, especially in highperformance applications such as nuclear reactors, electronics, and energy production and storage devices, the study and development of multilayered materials are expected to continue and expand [4].Multilayer materials' performance and their corrosion resistance can be influenced by the properties of individual layers, their interactions, and how they respond to environmental factors.The heterogeneity inherent in multicomponent and especially multilayered materials presents unique challenges, necessitating the development of tailored analytical techniques that can probe each layer in detail and provide a concentration profile.As the need for materials with customized properties grows, especially in high-performance applications such as nuclear reactors, electronics, and energy production and storage devices, the study and development of multilayered materials are expected to continue and expand [4].Multilayer materials' performance and their corrosion resistance can be influenced by the properties of individual layers, their interactions, and how they respond to environmental factors.The heterogeneity inherent in multicomponent and especially multilayered materials presents unique challenges, necessitating the development of tailored analytical techniques that can probe each layer in detail and provide a concentration profile.
Only a handful of analytical techniques can produce concentration profiles during measurements.This group includes methods in which ablation is integral to the analytical process.Ablation is commonly achieved using high-energy beams, such as those from lasers [8], ions [9], or high-energy electrons [10].Techniques such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) [11], laser-induced breakdown spectroscopy (LIBS), secondary ion mass spectrometry (SIMS) [12], and glow discharge optical emission spectrometry (GD-OES) [13] have been developed and are widely used across various laboratories.However, other methods, such as laser ionization mass spectrometry (LIMS) [14] and electron probe microanalysis (EPMA) [15] are less widespread.
Among the techniques mentioned earlier, LIBS stands out prominently [16,17].LIBS is an atomic emission spectroscopy method.It utilizes a high-intensity laser pulse to ablate a minute section of the sample.By consecutively ablating the sample with multiple laser pulses and subsequently analyzing the emitted spectrum with each pulse, it is possible to gather information about elemental concentration with respect to depth.Each laser pulse strips away a thin layer, enabling the subsequent pulse to explore a slightly deeper portion.The ejected matter from this process forms a plasma plume, emitting light that is representative of the elements within the sample.LIBS can offer qualitative and quantita-Scheme 1.Schematic representation of materials with a different distribution of components (blue and green squares represent atoms of two different elements).(a) Material with an even distribution of components (composite), (b) material with abruptly changed composition (multilayered material), and (c) material with continuous transition in composition (functionally graded or corroded materials).Illustrations prepared using the simulation performed using the NetLogo Models Library [7].
Only a handful of analytical techniques can produce concentration profiles during measurements.This group includes methods in which ablation is integral to the analytical process.Ablation is commonly achieved using high-energy beams, such as those from lasers [8], ions [9], or high-energy electrons [10].Techniques such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) [11], laser-induced breakdown spectroscopy (LIBS), secondary ion mass spectrometry (SIMS) [12], and glow discharge optical emission spectrometry (GD-OES) [13] have been developed and are widely used across various laboratories.However, other methods, such as laser ionization mass spectrometry (LIMS) [14] and electron probe microanalysis (EPMA) [15] are less widespread.
Among the techniques mentioned earlier, LIBS stands out prominently [16,17].LIBS is an atomic emission spectroscopy method.It utilizes a high-intensity laser pulse to ablate a minute section of the sample.By consecutively ablating the sample with multiple laser pulses and subsequently analyzing the emitted spectrum with each pulse, it is possible to gather information about elemental concentration with respect to depth.Each laser pulse strips away a thin layer, enabling the subsequent pulse to explore a slightly deeper portion.The ejected matter from this process forms a plasma plume, emitting light that is representative of the elements within the sample.LIBS can offer qualitative and quantitative assessments of the elemental composition through this emitted light analysis.Impressively, LIBS is sensitive to most elements in the periodic table.A comparative analysis of the elements detected and their respective detection limits when using the LIBS technique versus the more conventional XRF technique is represented graphically in a periodic table in Figure 3.The ablation crater's depth can be measured using optical or electron microscopy [18], atomic force microscopy [19], or profilometry [20,21].A depth profile of elemental concentration can be constructed by correlating the depth with the number of laser pulses and the gathered spectra.
Highlighting its adaptability, LIBS is apt for both stationary laboratory settings and on-site field applications [22].Stationary LIBS equipment is commercially available, and there are also portable variants [23,24], analogous to widely used portable XRF analyzers.A notable recent advancement in LIBS technology is an analyzer combined with an optical microscope [25].This integration facilitates elemental analysis while also allowing the visualization of sample details and measurement of craters created during the ablation process.
The development of the LIBS technique and its applications have been reviewed several times since 2013, when monumental work was published, covering 300+ articles [26].Since then, several review articles have been published each year dedicated to the characterization of the LIBS technique, the development of equipment, and its applications.The published data and review articles indicate that despite the potential of LIBS for depth profiling, its predominant application remains surface mapping [27].To emphasize the capability of LIBS to provide depth concentration profiles or stratigraphic analysis, a vital feature for multilayer materials, functionally graded materials, and diffusion-controlled processes, we have chosen to review LIBS-related articles published between 2008 and 2023.This time range allows us to trace the applications of the technique, ranging from relatively simple model samples of multilayer systems to its use in quality control performed on-site by thin-solar-cell manufacturers.
the elements detected and their respective detection limits when using the LIBS technique versus the more conventional XRF technique is represented graphically in a periodic table in Figure 3.The ablation crater's depth can be measured using optical or electron microscopy [18], atomic force microscopy [19], or profilometry [20,21].A depth profile of elemental concentration can be constructed by correlating the depth with the number of laser pulses and the gathered spectra.
Highlighting its adaptability, LIBS is apt for both stationary laboratory settings and on-site field applications [22].Stationary LIBS equipment is commercially available, and there are also portable variants [23,24], analogous to widely used portable XRF analyzers.A notable recent advancement in LIBS technology is an analyzer combined with an optical microscope [25].This integration facilitates elemental analysis while also allowing the visualization of sample details and measurement of craters created during the ablation process.
The development of the LIBS technique and its applications have been reviewed several times since 2013, when monumental work was published, covering 300+ articles [26].Since then, several review articles have been published each year dedicated to the characterization of the LIBS technique, the development of equipment, and its applications.The published data and review articles indicate that despite the potential of LIBS for depth profiling, its predominant application remains surface mapping [27].To emphasize the capability of LIBS to provide depth concentration profiles or stratigraphic analysis, a vital feature for multilayer materials, functionally graded materials, and diffusion-controlled processes, we have chosen to review LIBS-related articles published between 2008 and 2023.This time range allows us to trace the applications of the technique, ranging from relatively simple model samples of multilayer systems to its use in quality control performed on-site by thin-solar-cell manufacturers.The periodic table with elements determined by LIBS, XRF, and portable XRF (pXRF) is marked in green.Limits of detection (LOD) for LIBS and pXRF, expressed in µg/g, are noted below the respective element symbols.The LODs for stationary XRF are not provided due to the varied capabilities of different XRF instruments.The LODs for LIBS, specific to individual elements, are sourced from papers [33,34] with permission.The ability of LIBS to detect Br and I is also detailed in [35].The papers [36][37][38] present the LIBS spectra for noble gases: He, Ar, Xe, and Kr.Information on Np determination through LIBS is available in [39], while LODs for pXRF are obtained from [40] with permission.

Impact of Crater Formation on LIBS Signal and Profile Analysis
The initial step in LIBS analysis involves irradiating the sample material with a highintensity laser pulse.This action generates a microplasma above the sample and creates a distinct physical mark, an ablation crater.This crater offers valuable insights into the material's layered composition, ablation dynamics, and the details of laser-sample interactions.When the laser beam interacts with the sample, it sharply increases the surface temperature, leading to the vaporization of a small material segment and the subsequent creation of a crater.In the context of multilayered materials, the formation of craters in LIBS serves as a "drilling" mechanism, allowing for exploring deeper layers that are typically not accessible in surface analyses.The depth directly signifies the layers accessed and profiled, which makes its geometry critical for depth-specific analyses.The structure of the crater is influenced by multiple factors, including (i) the laser parameters (like pulse width, fluence, the energy density imparted to the material's surface during laser exposure, beam profile; focalization, etc.), (ii) the inherent properties of the material being analyzed, as certain elements can cause a sample to absorb more laser energy, leading to a deeper crater for the same laser parameters, and (iii) the surrounding atmospheric conditions (air, inert atmosphere, etc.) [8,16,17].The composition (matrix) can influence the depth and shape of the ablation crater and, consequently, the LIBS signal.For example,   [33,34] with permission.The ability of LIBS to detect Br and I is also detailed in [35].The papers [36][37][38] present the LIBS spectra for noble gases: He, Ar, Xe, and Kr.Information on Np determination through LIBS is available in [39], while LODs for pXRF are obtained from [40] with permission.

Impact of Crater Formation on LIBS Signal and Profile Analysis
The initial step in LIBS analysis involves irradiating the sample material with a highintensity laser pulse.This action generates a microplasma above the sample and creates a distinct physical mark, an ablation crater.This crater offers valuable insights into the material's layered composition, ablation dynamics, and the details of laser-sample interactions.When the laser beam interacts with the sample, it sharply increases the surface temperature, leading to the vaporization of a small material segment and the subsequent creation of a crater.In the context of multilayered materials, the formation of craters in LIBS serves as a "drilling" mechanism, allowing for exploring deeper layers that are typically not accessible in surface analyses.The depth directly signifies the layers accessed and profiled, which makes its geometry critical for depth-specific analyses.The structure of the crater is influenced by multiple factors, including (i) the laser parameters (like pulse width, fluence, the energy density imparted to the material's surface during laser exposure, beam profile; focalization, etc.), (ii) the inherent properties of the material being analyzed, as certain elements can cause a sample to absorb more laser energy, leading to a deeper crater for the same laser parameters, and (iii) the surrounding atmospheric conditions (air, inert atmosphere, etc.) [8,16,17].The composition (matrix) can influence the depth and shape of the ablation crater and, consequently, the LIBS signal.For example, certain elements can cause a system to absorb more laser energy, leading to a deeper crater for the same laser parameters.
In Figure 4, a gallery of craters resulting from laser ablation is presented to show the influence of various parameters on crater formation.Specifically, Figure 4a shows the effect of laser fluence, Figure 4b underscores the combined impact of laser fluence and the number of laser pulses, while Figure 4c presents the role of frequency.These illustrations are based on samples of silicone [41] and brass [18].However, the morphology of LIBS craters is also addressed in several other studies devoted to the analysis of copper galvanized with nickel [42], galvanized steel [43], brass [44], and archeological samples of ceramics [20].Typically, a higher laser fluence results in a larger crater, as illustrated in Figure 4a, using silicon and brass as examples.As laser fluence increases, so does the diameter of the crater [45].During ablation, the boiling material can cause splashes or splatters around the crater due to the expansion of the plasma, which creates a recoil pressure.These "splashes" of ablated material can distribute portions of the examined multilayered sample outside and inside the crater.This can alter the original material, affecting surface mapping (as shown in Figure 5) and depth profiling [46].Such alterations are particularly significant when the distance between sampling points is minimal [47,48].In the case of polymeric materials, there is an additional challenge associated with the high viscosity of the material produced by melting due to the action of the laser beam.An increase in the molecular weight of the polymer has been reported to lead to a decrease in the ablation rate of the polymer [49].
The typical depth resolution for LIBS depth profiling falls between 100 and 500 nm [46], although craters reaching several tens of micrometer depths [21,50,51] or even thousands of micrometer have also been documented [52].The ablation rate is influenced by the type of material that is being ablated and the laser fluence (as shown in Figure 6a,b [41,53]), as well as by the sample temperature [21].For consistent material and ablation conditions, the depth increases proportionally with fluence [53] and the number of laser pulses (see Figure 6b,c) [41,44].If the material removal rate remains consistent at a given fluence, one can correlate the depth with the number of laser pulses (as depicted in Figure 6b) [41], but for deep ablation the linear range of crater depth vs. number of pulses dependance is limited as shown for aluminum [44] and superalloy samples (Figure 6c) [52].This correlation facilitates the construction of elemental concentration depth profiles or allows for the calculation of the thickness of the material layer [54].Although this assumption tends to be accurate for homogeneous materials and relatively shallow depths, it may oversimplify scenarios involving multilayered structures [46].In such cases, examining the morphology and consistency of LIBS-generated craters becomes invaluable.Such studies can provide information on the varying ablation thresholds and behaviors exhibited by the different layers within a multilayered sample.Understanding and managing the dynamics of LIBS craters allows one to optimize the technique variables to provide precise depth profiles. (c)

LIBS Spectral Lines
During LIBS measurement, a laser pulse vaporizes a minute portion of the material, forming a microplasma.As this microplasma cools, the excited ions and atoms transition to their ground states.This shift from higher to lower energy levels results in the emission of photons with specific wavelengths, which are indicative of the elements in the sample.[41] with permission).(c) Dependence of the crater depth on the number of laser pulses for the superalloy (adapted according to [52] with permission).

LIBS Spectral Lines
During LIBS measurement, a laser pulse vaporizes a minute portion of the material, forming a microplasma.As this microplasma cools, the excited ions and atoms transition to their ground states.This shift from higher to lower energy levels results in the emission of photons with specific wavelengths, which are indicative of the elements in the sample.These emitted photons, ranging in wavelengths from ultraviolet to microwave, are subsequently collected and spectrally dispersed by a spectrometer.Figure 7 shows exemplary spectra across these wavelengths.The distinct spectral "fingerprint" rendered by LIBS allows for both qualitative and, with proper calibration, quantitative identification of elements in the analyzed material.It should be noted that, as shown in Figure 7b,c, LIBS reveals not only atomic bands, but also molecular bands [55,56], such as those produced by CN (cyanogen) and C 2 (swan bands), OH [57] or AlO [58].In the LIBS process, the characteristic emission bands of CN become apparent when carbon reacts with nitrogen in the generated plasma.These bands serve as a valuable tool for detecting materials containing carbon.If the CN band fades after the initial LIBS laser pulse, it suggests that the sample was coated with a layer containing carbon compounds, as described in the antique bronze sculpture [59].Such observations can act as indicators of surface contamination or be used to evaluate the effectiveness of surface cleaning procedures [59,60].In particular, these molecular bands, including C 2 and CN, are frequently used in polymer studies [61][62][63].
ferent fluences (adapted according to [41] with permission).(c) Dependence of the crater depth on the number of laser pulses for the superalloy (adapted according to [52] with permission).

LIBS Spectral Lines
During LIBS measurement, a laser pulse vaporizes a minute portion of the material, forming a microplasma.As this microplasma cools, the excited ions and atoms transition to their ground states.This shift from higher to lower energy levels results in the emission of photons with specific wavelengths, which are indicative of the elements in the sample.These emitted photons, ranging in wavelengths from ultraviolet to microwave, are subsequently collected and spectrally dispersed by a spectrometer.Figure 7 shows exemplary spectra across these wavelengths.The distinct spectral "fingerprint" rendered by LIBS allows for both qualitative and, with proper calibration, quantitative identification of elements in the analyzed material.It should be noted that, as shown in Figure 7b,c, LIBS reveals not only atomic bands, but also molecular bands [55,56], such as those produced by CN (cyanogen) and C2 (swan bands), OH [57] or AlO [58].In the LIBS process, the characteristic emission bands of CN become apparent when carbon reacts with nitrogen in the generated plasma.These bands serve as a valuable tool for detecting materials containing carbon.If the CN band fades after the initial LIBS laser pulse, it suggests that the sample was coated with a layer containing carbon compounds, as described in the antique bronze sculpture [59].Such observations can act as indicators of surface contamination or be used to evaluate the effectiveness of surface cleaning procedures [59,60].In particular, these molecular bands, including C2 and CN, are frequently used in polymer studies [61][62][63].Table 1 provides an overview of the multicomponent samples analyzed using LIBS.These samples span a wide range of materials tailored for extreme environments, such as
The study of functionally graded materials further highlights the effectiveness of LIBS in examining elemental concentration profiles.With LIBS, it was feasible to identify the presence of single-component layers and, more critically, determine the extent of a transitional layer with a composition influenced by diffusion.Figure 2c offers a vivid illustration of LIBS's capabilities in analyzing systems with varied compositions.Furthermore, LIBS proves to be instrumental in studying corrosion processes.As illustrated in Figure 2d, LIBS enables not only the identification of the composition of the corroded layer but also an assessment of its reach.
The application of LIBS for depth profiling in polymeric materials is rare (see Table 2).However, this trend may shift soon.As suggested by a study from 2020 [30], LIBS can effectively investigate multilayer polymeric materials, such as that shown in Figure 1c.The concentration profiles obtained using LIBS for multilayer materials depicted in Figure 1d-f, coupled with microscopic images of the sample analyzed, confirm that the LIBS technique is highly effective in the study of multilayer materials, as all individual layers of various compositions were precisely identified [15].However, achieving accurate depth profiling requires the appropriate calibration and optimization of the measurement parameters.  1 the reference data were obtained using the EDS, XPS, and SIMS techniques.Explanation of abbreviations: acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), polyethylene (PE), polyacrylate (PAK), and polyvinylchloride (PVC).

Analytical Performance of LIBS for Elemental Profiling
When constructing elemental depth profiles of multilayered materials, the dependence of the intensity of the spectral line of interest is typically plotted against the number of laser pulses (as shown in Table 1).Concentration profiles were reported in fewer than 20% of the experiments described in Tables 1 and 2. For calibration, classic calibration curve methods were predominantly used [75][76][77]80], although the calibration-free approach (CF-LIBS) was also applied in its traditional [81] and modified versions (one-line calibration-free LIBS, OLCF-LIBS) [89].Despite its undeniable benefits, the internal standard method was used in a single study [79].Multivariate statistical methods, such as principal component analysis (PCA) and clustering of K-means, have proven to be highly effective in determining the content of individual components in multilayered materials [30].In the following, a brief overview of the challenges and considerations associated with quantitative analysis using LIBS is presented.These include aspects related to calibration, matrix effects, instrumental parameters, and the complexities of interpreting LIBS spectra.

Classic and Calibration-Free LIBS
As indicated by the studies presented in the aforementioned works, quantitative elemental analysis performed using LIBS is a challenging task because of the multiple factors that influence the intensity of a LIBS signal.These range from laser-specific parameters like pulse energy, duration, wavelength, and the distance between the laser and the sample to properties of the sample's surface, such as contamination levels and surface roughness.Additionally, the composition of the sample matrix can also introduce spectral interference, making calibration quite complicated.
However, the right calibration approach can mitigate these challenges and lead to accurate results.Traditional calibration strategies, such as the construction of calibration curves based on external standards, are often employed.Many elements exhibit numerous LIBS emission lines with distinct intensities.As a result, these lines can produce calibration curves with varied slopes.For instance, several linear regression calibration curves for phosphorus [100], chromium [101], and cerium [102] have been documented.Beyond single-variable calibration, multivariable regression models can be used that incorporate multiple signals from the LIBS spectrum.Different calibration variants, some incorporating chemometric tools, are discussed in reviews such as [102].When choosing spectral lines for calibration, it is crucial to ensure that the chosen line is free from interference from coexisting elements.For example, although the Al I line at 309.27 nm shows a higher intensity, it is not preferred for aluminum determination due to the potential interferences of Mg I lines at 309.10, 309.29, and 309.68 nm [100].Spectrum processing methods, such as smoothing, can enhance the shape of the calibration curve [101].
However, the use of calibration curves, even those constructed with reference materials that have a matrix composition similar to that of the sample, may not always be sufficient, especially for specific materials, such as geological samples [101].In such situations, the incorporation of an internal standard method is beneficial.When experimental parameters change, the ratio of spectral line intensities is applied for calibration.This approach was presented for analysis of a thin solar cell film, where the In/Cu, Ga/In, and Se/In intensity ratios were used for calibration and finally concentration depth profiling [76].
In scenarios where traditional calibration is unfeasible, such as in remote-sensing applications in space missions, or when suitable standards are inaccessible or difficult to formulate (as in the analysis of unknown or complex materials), employing calibration-free LIBS (CF-LIBS) seems to be a promising solution.In CF-LIBS, the analyte concentration is calculated directly from the LIBS spectrum by relying on the physics of the laser-induced plasma.The method involves calculating the plasma temperature and electron density directly from the spectrum.These parameters, combined with atomic data, facilitate the determination of the analyte's concentration.However, the successful application of CF-LIBS necessitates the fulfillment of specific criteria: (i) congruent mass transfer from the solid to the plasma, (ii) local thermodynamic equilibrium within the plasma, (iii) spatially uniform plasma temperature and density distributions, and (iv) negligible self-absorption of spectral lines.If these conditions are met, then the measured line intensity is proportional to the emission coefficient [103].Utilizing an array of spectral lines corresponding to varied energy levels enables calculation of each sample element's concentration without conventional calibration.Achieving accurate relative concentrations with CF-LIBS is difficult due to various influencing factors, such as matrix effects, laser parameters, and experimental configurations.To mitigate these problems, several adaptations of CF-LIBS have been proposed [103].Recent review publications [104,105] provide a detailed discussion of foundational concepts, their influencing factors, analytical efficacy, and potential measurement uncertainties.
An interesting variant of CF-LIBS worth mentioning is the one-point calibration method.Using only one calibration point, it introduces an empirical correction to the purely theoretical CF-LIBS approach, helping to determine essential experimental and spectroscopic parameters, which are normally not easy to obtain [104].

Limits of LIBS Detection of LIBS
The limit of detection (LOD) in elemental analysis is the minimum elemental fraction for which the analytical signal can be distinguished from the background signal within a stated probability.Assuming a normal distribution of the background signal fluctuation, the limit of detection within a confidence interval of 99.7% is given by Equation (1) [106].
where σ is the standard deviation of the background signal and S is the sensitivity given by the slope of the calibration curve.The calculated limits of detection (LOD) for most elements are shown in Figure 3.Although determining the LOD may seem straightforward at first glance, in the context of LIBS it is a complex task [107].The complexities arise from various factors, including the calibration methods (univariate, multivariate) [107] and the problems specific to the calibration-free LIBS approach [104,108].

Conclusions
Laser-induced breakdown spectroscopy has emerged as a promising technique for the rapid, non-destructive, or micro-destructive depth profiling of multilayer materials with well-defined layers and diffusion-driven materials.As this review has shown, with its ability to produce elemental maps and profiles, LIBS offers an unparalleled tool for studying the surface and deeper layers of multilayer materials.When LIBS results are confronted with traditional microscopic techniques, there is clear evidence of the reliability of the technique.LIBS has showcased a broad applicability for multilayer organic and inorganic materials, functionally graded materials, and materials affected diffusion-controlled processes such as corrosion.However, like any analytical tool, the effectiveness of LIBS hinges on the optimal calibration and fine-tuning of the measurement parameters.

Future Directions in LIBS Applications
As technology evolves, it can be anticipated that more sophisticated LIBS equipment will be introduced that may reduce the need for extensive multipoint calibration by employing calibration free approach, offer a higher spatial resolution, or introduce faster data acquisition speeds.Analyzing the data presented in Table 1, it can be expected that the applications of LIBS in multilayered materials research will evolve following the directions indicated below.

1.
Remote measurements.In LIBS measurements, the analytical information comes from radiation emitted by elements in plasma, not from particles reaching the detector, as is the case, for example, with LA-ICP-MS.This feature enables remote analysis, a feature currently utilized in the study of the Martian surface.Remote analysis is invaluable for investigating materials that operate under extreme conditions or for situations where direct contact with the material would expose the operator to harmful radiation.Future applications are expected to include the use of LIBS to monitor the progress of wear and corrosion in various types of materials used for the construction of nuclear reactors.

2.
Industrial at-line applications.LIBS measurements do not require vacuum chambers or direct contact with the sample, provide data on elemental composition quickly, and are easily automated.This makes the technique a promising tool for quality control in industries that use multilayered materials in the production of displays, monitors, thin-film photovoltaic cells, coatings, and graded wear-and heat-resistant materials.

3.
Identification of materials of unknown composition.The use of calibration-free LIBS allows the identification and quantification of elements in materials with unknown compositions.Waste management, particularly recycling processes, is an anticipated application area for LIBS.This is especially crucial for multilayer materials, where valuable or toxic constituents might be embedded, making them inaccessible for surface-only analyses.By creating a depth profile via the ablation of micro-areas within the waste, LIBS facilitates the confirmation or elimination of undesirable components.This eliminates the need for costly and energy-consuming sample fragmentation, a particularly challenging process with polymeric or composite materials.Such materials include laminates, substances layered with organic and inorganic coatings, electronic waste, used photovoltaic panels, and even microplastics [109].4.
Development of integrated analytical platforms.Hybrid systems where LIBS is integrated with other analytical tools (e.g., Raman-LIBS systems, microscopes coupled with LIBS as analogous to SEM-EDS systems) could be developed to provide complementary data, enhancing the analytical capability of single-mode devices. 5.
Development of laser cleaning devices equipped with a LIBS analyzer.LIBS analyzers can be integrated with laser surface cleaning devices, as laser radiation used to remove the outer layers of material can also serve as the radiation inducing the LIBS effect.This enables real-time monitoring of the efficiency of removing layers with varied chemical compositions by tracking changes in an indicator element specific to the ablated material.This capability can be utilized to track the removal of external layers during the process of ablation of worn protective coatings in aviation and seems to be usable in the cleaning of historical artifacts.

Scheme 1 .
Scheme 1. Schematic representation of materials with a different distribution of components (blue and green squares represent atoms of two different elements).(a) Material with an even distribution of components (composite), (b) material with abruptly changed composition (multilayered material), and (c) material with continuous transition in composition (functionally graded or corroded materials).Illustrations prepared using the simulation performed using the NetLogo Models Library[7].Some examples of multilayer and diffusion-controlled materials are presented in Figures 1 and 2.As the need for materials with customized properties grows, especially in high-performance applications such as nuclear reactors, electronics, and energy production and storage devices, the study and development of multilayered materials are expected to continue and expand[4].Multilayer materials' performance and their corrosion resistance can be influenced by the properties of individual layers, their interactions, and how they respond to environmental factors.The heterogeneity inherent in multicomponent and especially multilayered materials presents unique challenges, necessitating the development of tailored analytical techniques that can probe each layer in detail and provide a concentration profile.Only a handful of analytical techniques can produce concentration profiles during measurements.This group includes methods in which ablation is integral to the analytical process.Ablation is commonly achieved using high-energy beams, such as those from lasers[8], ions[9], or high-energy electrons[10].Techniques such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)[11], laser-induced breakdown spectroscopy (LIBS), secondary ion mass spectrometry (SIMS)[12], and glow discharge optical emission spectrometry (GD-OES)[13] have been developed and are widely used across various laboratories.However, other methods, such as laser ionization mass spectrometry (LIMS)[14] and electron probe microanalysis (EPMA)[15] are less widespread.Among the techniques mentioned earlier, LIBS stands out prominently[16,17].LIBS is an atomic emission spectroscopy method.It utilizes a high-intensity laser pulse to ablate a minute section of the sample.By consecutively ablating the sample with multiple laser pulses and subsequently analyzing the emitted spectrum with each pulse, it is possible to gather information about elemental concentration with respect to depth.Each laser pulse strips away a thin layer, enabling the subsequent pulse to explore a slightly deeper portion.The ejected matter from this process forms a plasma plume, emitting light that is representative of the elements within the sample.LIBS can offer qualitative and quantita-

Figure 2 .
Figure 2. (a) SEM image of functionally graded material, a thermal barrier coating used in the manufacturing of jet engines (adapted according to [31] with permission).(b) Photograph of ancient bronze rings (adapted according to [32] with permission).(c,d) Concentration profiles obtained for functionally graded material (c) and ancient rings (d) marked with an arrow in panel b (adapted according to [31,32] with permission).

Figure 3 .
Figure3.The periodic table with elements determined by LIBS, XRF, and portable XRF (pXRF) is marked in green.Limits of detection (LOD) for LIBS and pXRF, expressed in µg/g, are noted below the respective element symbols.The LODs for stationary XRF are not provided due to the varied capabilities of different XRF instruments.The LODs for LIBS, specific to individual elements, are sourced from papers[33,34] with permission.The ability of LIBS to detect Br and I is also detailed in[35].The papers[36][37][38] present the LIBS spectra for noble gases: He, Ar, Xe, and Kr.Information on Np determination through LIBS is available in[39], while LODs for pXRF are obtained from[40] with permission.

Figure 3 .
Figure3.The periodic table with elements determined by LIBS, XRF, and portable XRF (pXRF) is marked in green.Limits of detection (LOD) for LIBS and pXRF, expressed in µg/g, are noted below the respective element symbols.The LODs for stationary XRF are not provided due to the varied capabilities of different XRF instruments.The LODs for LIBS, specific to individual elements, are sourced from papers[33,34] with permission.The ability of LIBS to detect Br and I is also detailed in[35].The papers[36][37][38] present the LIBS spectra for noble gases: He, Ar, Xe, and Kr.Information on Np determination through LIBS is available in[39], while LODs for pXRF are obtained from[40] with permission.

Figure 4 .Figure 4 .
Figure 4. (a-c) SEM images depicting the effects of laser ablation on silicon (a,c) (adapted according to [41] with permission) and brass (b) (adapted according to [18] with permission) substrates under varied conditions: (a) variation in laser fluence, (b) combined variation in laser fluence and a number of laser pulses, and (c) combined variation in repetition rate and number of laser pulses.

Figure 4 .Figure 5 .
Figure 4. (a-c) SEM images depicting the effects of laser ablation on silicon (a,c) (adapted according to[41] with permission) and brass (b) (adapted according to[18] with permission) substrates under varied conditions: (a) variation in laser fluence, (b) combined variation in laser fluence and a number of laser pulses, and (c) combined variation in repetition rate and number of laser pulses.

Figure 5 .Figure 6 .
Figure 5. (a,c) SEM images showing a single ablation crater formed on Cu (a) [47] and on multilayer Al-Ni-Fe samples (c), where the colors indicate the abundance of the elements Al (violet), Ni (green)and Fe (red) (adapted according to[48] with permission).(b,d) Array of ablation craters formed on Cu (b) (adapted according to[47] with permission) and Al-Ni-Fe samples (d)[48].

Figure 6 .
Figure 6.(a) Dependence of the ablation rates on the laser fluence for Al (h Al ), W (h w ), and Mo (h Mo ); AAR corresponds to the average ablation rate (adapted according to [53] with permission).(b) Dependence of the depth of crater ablation of crystalline silicon on the number of laser pulses at different fluences (adapted according to[41] with permission).(c) Dependence of the crater depth on the number of laser pulses for the superalloy (adapted according to[52] with permission).

Table 1 .
List of applications of the LIBS technique in the studies of inorganic multilayer materials, functionally graded materials, and corrosion studies employing commercial LIBS instruments (both stationary and portable) and laboratory-made LIBS systems.

Table 2 .
List of applications of the LIBS technique in the studies of polymeric multilayer materials and corrosion studies of polymers employing commercial LIBS and laboratory-made LIBS systems.