On the Similarity and Differences Between Nano-Enhanced Laser-Induced Breakdown Spectroscopy and Nano-Enhanced Laser-Induced Plasma Spectroscopy in Laser-Induced Nanomaterials Plasma

Abstract

The interaction of pulsed lasers with matter involving nanomaterials as a pure target or thin layer deposited on a target initiates transient plasma, which shows strong enhancement in a spectral line emission. This domain of research has been explored via two well-established techniques dubbed NELIBS and NELIPS. These Nano-Enhanced Laser-Induced Breakdown or Plasma Spectroscopy techniques entail similarities as well as differences. The newly defined concept of Nano-Enhanced Laser-Induced Plasma Spectroscopy NELIPS is introduced. Thereupon, certain confusion has arisen from various aspects of the similarities as well as differences between the two techniques. In this article, we will investigate the application of either technique to retrieve relevant data about the enhanced spectral line plasma emission phenomenon. To discriminate between these two techniques, a survey on the nature of the target, the origin of enhancement and prevalent theoretical approaches is presented. In this context, the potential achievements, challenges and expected prospects are comparatively highlighted. This review emphasizes the unique contributions of NELIPS, particularly the advanced approach in nanoscale thermal modeling and spectroscopic applications.

1. Introduction

1.1. Plasma from the Thermodynamical Point of View

Plasma is the fourth state of matter after solid, liquid and gas states [1,2,3,4,5]. Thermodynamically, it can be regarded as an ensemble that comprises four different types of species. These include three corpuscular atoms, ions and electrons, while the fourth component is radiation. It exists at relatively large temperatures; therefore, a small fraction of atoms is typically ionized, creating a finite number of ions and electrons [1,2,3,4,5]. Some of these electrons escape from the plasma active volume while the residue constitutes a cloud of quasi-free electrons oscillating in a collective manner about the heavy-positive ionic background [1,2,3,4,5]. Each kind of species maintains a local thermodynamical equilibrium with its own species, and hence, each one component species is distributed according to its proper equilibrium distribution function characterized by a certain temperature [1,2,3,4,5,6,7]. A compact summary of the proper equilibrium distribution functions correlated with each of the plasma constituents is given in

Table 1. Proper set of equilibrium distribution functions for each one of the plasma species. The meaning of different symbols can be found in the list of symbols.

Species	Proper Distribution	Proper Expression
Atoms	Boltzmann	${\displaystyle \frac{N_j}{g_j}}={\displaystyle \frac{N_0}{U_0}}\exp \left({\displaystyle \frac{-E_j}{K{T}_a}}\right)$
Electrons	Maxwell	$f\left(v\right)dv=4\pi v^2{\left({\displaystyle \frac{m_e}{2\pi K{T}_e}}\right)}^{-1.5}\exp \left({\displaystyle \frac{-m_e{v}^2}{2K{T}_e}}\right)dv$
Ions	Saha–Boltzmann	${\displaystyle \frac{N_n^{+}}{g_n}}={\displaystyle \frac{n_e{N}_0^{+}}{2U_0^{+}}}{\left({\displaystyle \frac{h^2}{2\pi m_{e}K{T}_i}}\right)}^{-1.5}\exp \left({\displaystyle \frac{{\epsilon }_i-\Delta {\epsilon }_i-E_n^{+}}{2K{T}_i}}\right)$
Radiation	Planck	$I\left(\lambda ,T_R\right)={\displaystyle \frac{2h{c}^2}{{\lambda }^5}}{\left(\exp \left({\displaystyle \frac{hc}{\lambda K{T}_R}}\right)-1\right)}^{-1}$

.

It is common practice that plasma can be fully characterized on account of the values of two measurable parameters, namely, electron density $n_e$ and temperature $T_e$ [1,2,3,4,5,6,7,8,9,10,11,12,13]. Both should be measured experimentally with sufficient accuracy in a process called plasma diagnostics. The values of these two parameters determine the thermodynamical state of plasma and, hence, the valid set of proper distribution functions that can be usefully applied to the plasma ensemble. In plasmas, the collisional processes, rather than radiative ones, control the thermodynamic state [2,3,4,5,6,7,9,11,13]. The crucial parameter is the electron–atom collision frequency, which is correlated with plasma parameters via the relation: $\left(f_c\propto n_e{T}_e^{-1.5}\right)$ [2,3,5,6,7,8,9].

Briefly, the electron density determines the plasma state of equilibrium, while the electron temperature vitally determines the strength of the variation in these distribution functions. In

Table 2. The plasma states of equilibrium at different ranges of electron density and implicit conditions at different temperatures.

Electron Density $({\mathbf{cm}}^{-3})$	State of Equilibrium	Conditions on Temperatures	Applicable Distribution Functions
$n_e\ge {10}^{18\text{–}19}$	Complete Thermodynamical Equilibrium (CTE)	$T_a\simeq T_e\simeq T_i\simeq T_R$	Boltzmann; Saha–Boltzmann Maxwell; Planck
${10}^{19\text{–}18}\ge n_e\ge {10}^{16\text{–}15}$	Local Thermodynamical Equilibrium (LTE)	$T_a\simeq T_e\simeq T_i\ne T_R$	Boltzmann; Saha–Boltzmann Maxwell
${10}^{16\text{–}15}\ge n_e\ge {10}^{11\text{–}10}$	Partial Local Thermodynamical Equilibrium (PLTE)	$T_a\simeq T_e\ne T_i\ne T_R$	Boltzmann Maxwell
$n_e\le {10}^9$	Corona State (Equilibrium)	$T_a\ne T_e\ne T_i\ne T_R$	None of these distribution functions is applicable, and collisional–radiative modeling should be constructed.

, a compact list of the dominant states of the equilibrium of plasma is presented together with the approximate value ranges of electron density and the valid set of equilibrium distribution functions [1,2,3,4,5,6,7,8,9]. Accurate analysis of the emitted light from plasma in the so-called optical plasma spectroscopy OES technique is usually conducted, assuming that the emitted light is sufficiently influenced by plasma parameters [2,3,4,5,6,7,8,9]. Fortunately, plasmas produced by the interaction of pulsed lasers with matter are often in the state of the LTE [2,3,4,5,6,7,8,9,10,11], characterized by a density range from 10¹⁸ to 10¹⁶ cm⁻³ and a temperature range from a few eV to sub-eV, except in special cases. However, as early as a few tenths of a nanosecond after the termination of the laser pulse and very close to the target, the plasma becomes hot enough that the plasma state can be close to the state of the CTE [2,3]. These equilibrium distributions cannot be maintained at a very large delay time (10th of μs) and/or at a relatively large distance from the target [2,3,7,9], at which the plasma becomes cool enough that no one of these distributions is any more valid, and a collisional–radiative mode should be constructed.

1.2. Plasma Spectroscopy (Optical Emission Spectroscopy—OES Technique)

Experimentally, light from plasma is resolved into its inherent spectral contents using versatile monochromators, spectrometers and/or spectrographs with suitable resolving power. These dispersive systems have recently been equipped with a suitable detector, usually a charged couple device CCD camera. Readily, time-controlled intensified ICCD cameras are now at hand, which convert optical signals into digital electronic pixel data on a graphical output reading unit. Analog-to-Digital ATD interfaces and/or controllers should be employed in conjunction with a compatible PC. Suitable software can routinely control and correlate the functional dependence of the emitted spectral radiance (or intensity) on the wavelength at the readout unit. The whole experimental setup should be synchronized. Wavelengths should be calibrated using standard low-pressure lamps and, if possible, absolutely calibrated using a standard radiometric light source [2,11].

Theoretically and eventually, the careful processing of plasma-emitted spectra helps glean useful information about the values of plasma parameters $\left(n_e,T_e\right)$, and one can specify the thermodynamic state of plasma, as given in Table 2, and, consequently, the applicable distribution function(s) according to Table 1. Nevertheless, the application of the proper distribution allows for theoretically predicting some important quantities, e.g., the population density of atomic ionic states $\left(N_j,N_n^{+}\right)$ as well as the population density of the ground states of atoms and ions $\left(N_0,N_0^{+}\right)$. Consequently, the theoretical value of the spectral line intensity can be expressed as

$$I_{Th}\left(\lambda ,T_e\right)=\left(hc{g}_j{A}_j/4\pi {\lambda }_j\right){\left(N_j/g_j\right)}^{B-SB}$$

(1)

where ${\left(N_j/g_j\right)}^{B-SB}$ denotes the population density per unit statistical weight of the upper emitting state (denoted j). This spectral intensity can be expressed in terms of the Boltzmann function and/or the Saha–Boltzmann distribution in the case of the inclusion of ionic lines in addition to neutral atomic lines. In addition, the direct comparison with the corresponding experimentally measured spectral line intensity $I_{Exp}\left(\lambda ,T_e\right)$ permits assessing the deviation of the plasma state from equilibrium. These procedures provide the basic investigation of the basic science behind laser induced plasma spectroscopy (LIPS) and/or laser induced breakdown spectroscopy (LIBS).

On the other hand, the target material ingredients, as well as the relative concentration of the different elements in the target matrix, can be obtained through elemental analysis correlated with intrinsic emission spectral lines [12]. This pertains as well to the domain of analytical spectrochemistry using laser-induced breakdown spectroscopy (LIBS) with its versatile range of applications. Quantitatively, the LIPS technique offers different methods to measure the plasma electron density and temperature [2,7,14]. One of the simplest methods to measure the electron density is based on the direct measurement of the width of the Lorentzian component of the emitted spectral line or the so called Stark full width at half maximum—FWHM $\left(\Delta {\lambda }_{Stark}\right)$. This spectral width can be theoretically evaluated using different theoretical approaches. One important approach is based on the impact approximation (perturbation theory) [8,9,13], which predicts the direct relation of plasma electron density to $\left(\Delta {\lambda }_{Stark}\right)$, e.g., for the non-hydrogenic atoms $\left(n_e\propto \Delta {\lambda }_{Stark}\right)$ [2,7,8,9,13]. The complete description of the different methods to measure electron density is out of the scope of this article and can be found in many published articles in the literature [2,3,4,5,6,7,8,9,10,11,12,13].

On the other hand, the electron temperature $\left(T_e\right)$ can be measured in terms of the relative intensity of two or more (optically thin) lines belonging to the same ionization stage with a large difference in excitation energy. In general, the Boltzmann and/or Saha–Boltzmann plot methods should be constructed, which would reproduce a perfect straight line with a negative slope reminiscent of the electron temperature [2,3,4,5,6,7,8,9,10,11,12,13]. This would require good knowledge of the inherent atomic quantities, such as the energy of the excited state, statistical weight and transition probability, as well as Stark broadening parameters, which can be found in different standard international tables [8,9,12,13].

Unfortunately, from the point of view of plasma spectroscopy, the spectral lines emitted from plasma produced by the focusing of pulsed laser light on targets exhibit various distortions, which lead to large errors in the measured values of plasma parameters. One of them is the distortion caused by laser-induced plasma inhomogeneity, which manifests itself via self-absorption (SA) [12,13] and/or self-reversal (SR), as well as spectral line asymmetry (see Appendix A). The first tends to increase the spectral line intensity and enlarge the experimentally measured FWHM, while the second is characterized by a clear dip that appears pronounced at the un-shifted transition wavelength (Appendix A). Corrections against self-absorption can be made by utilizing the benchmark (optically thin) H_α-spectral line at 656.27 nm [11], while the effect of self-reversal probably needs more effort to treat (see Appendix A). Additionally, a very confusing problem exists concerning the deviations of the wavelength at the peak emission from the tabulated standard value of the transition wavelength. This shift proves beneficial to physicists because they can use it to measure the plasma electron density (the amount of the spectral line shift depends on the electron density) [8,9,13]. Nevertheless, there is a relatively large uncertainty and/or “sometimes” absence of certain lines’ transition properties, e.g., the transition probability and/or Stark broadening parameters, which also preclude reliable results. Meanwhile, when the target material contains a complex matrix of elements of unknown ratios (matrix effect), the situation tends to be exacerbated. This pesky recurrent overlapping of two or more lines that originated from different elements at nearly the same wavelength poses a real problem in achieving a fair resolution by spectrometers [15]. In such cases, researchers feel uncertain about the existence (or absence) of some elements while diagnosing some complex matrixes, like elemental analysis of malignant tissues [14], identifying certain elements of unknown meteorites [16] or in forensic investigations [17], as well as recently discovered natural mines by geologists [18].

Moreover, one major problem concerning the LIBS technique is the limited level of the limit of detection (LOD). This would ultimately prevent differentiating the signal originating from minor concentration elements [19]. Normally, the LOD of the LIBS technique is typically around a few ppm (part per million) [20]. A limited improvement in the LOD could be achieved with the help of expensive techniques, e.g., the double-pulse laser technique (with different configurations) [21,22] and even with the use of femtosecond laser pulses [23].

Therefore, there was an urgent need for a new idea or relatively cheap technique that could enhance weak signals emerged from elements of a small concentration. This was achieved by utilizing the unique properties of nanomaterials [24,25]. Not very recently, a strong enhanced emission was observed from laser-produced plasmas of pure nanomaterials targets [24] or metallic samples covered by thin layers of noble nanomaterials (Ag, Au) [25]. Meanwhile, a series of published articles have investigated the role of nanomaterials in enhanced emission from plasmas and, in turn, an improvement in the LOD [24,25].

1.3. Enhanced Emission from Plasmas Induced by Laser Interaction with Nanomaterials

Nanomaterial is a class of materials with a maximum geometric size lower than 100 nm. This idea was initiated after the famous talk by R. Feynman [26], who indicated that, as one goes by matter size from atomic to nano dimensions, the physical properties should show peculiar changes.

Thereafter, the physical and chemical properties of such classes of materials showed substantial changes [27]. These changes were attributed to the large ratio of the surface area to volume [28], which leads to an increase in the number of atoms at the surface with respect to the total number of atoms in the nanoparticle. Also, being directly exposed to external effects, these atoms maintain quantum effects [29] as well as higher surface energy to sustain the particle shape [30]. Indeed, noticeable changes in thermal, mechanical, electrical and magnetic properties and, consequently, the optical properties of matter show up drastically upon reaching such small dimensions [31].

The first attempt to employ nanoparticles in the process of enhanced emission using laser induced breakdown spectroscopy (LIBS) was reported in 2009AD, by Ohta et al. [32] after the addition of a thin layer of Au and Ag nanoparticles to the surface of plant leaves. The observed enhanced emission from plant leaves was attributed (at that time) to the process of localized surface plasmon resonance (LSPR) without further details or a clear mechanism of enhancement. This phenomenon has been extensively studied under controlled laboratory conditions via the deposition of a thin layer of gold (Au) and silver (Ag) nanomaterials to a variety of tested samples in 2013AD by De Giacomo et al. [25]. Accordingly, this process was renamed, for the first time, as Nano-Enhanced Laser Induced Breakdown Spectroscopy NELIBS. A strong enhanced emission was reported from the surface of metallic, non-metallic and organic materials after depositing a thin layer of different nanomaterials in a variety of published articles under the acronym NELIBS [32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49]. This observed enhanced emission under the title of NELIBS was simply ascribed to the occurrence of resonance between the localized surface plasmons (LSPR) of nanoparticles with the frequency of incident laser light.

On the other hand, the first reported enhancement from different pure nanomaterials in comparison to corresponding bulk counterpart was communicated at 2012AD by EL Sherbini et al. [24]. Therefore, and for discrimination reasons, the measured enhanced emission from pure nanomaterials targets upon irradiation by pulsed lasers will pertinently considered “the first of its type” to be renamed as Nano-Enhanced Laser Induced Plasma Spectroscopy NELIPS.

It is worth noting that both approaches (NELIPS and NELIBS) still share a similar basic experimental setup (OES technique) and call for the same procedures of spectroscopic measurements, which attest to the strong enhanced emission from laser induced plasmas utilizing nanomaterials.

In this context, however, an overall review of publications about NELIBS and NELIPS reveals some essential basic differences regarding the nature of target materials (a pure nanomaterial or the substrate material covered with thin layer of nanomaterial) and the origin of enhancement (from the pure nanomaterial or from the substrate material). In addition, there remain the different aspects of tackling the theoretical approach from the point of view of the physics of the pulsed laser interaction process or in the field of applications, the recommended basic theory (thermal processes or the foundations of EM theory) and the achievements or advantages of each approach, as well as challenges or limitations and, finally, the expected prospects of both in the future.

2. Materials and Methods in NELIBS and NELIPS

Schematic diagrams of the basic experimental setup and detection technique are described in detail in the literature dealing with NELIBS [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49] or NELIPS [24,50,51,52,53,54,55,56]. These incorporate almost the same basic layout except for the heterogeneous fabric of the targets under investigation in NELIBS, as well as the specific devices or laser facilities used by different researchers. The usual procedures for the spectroscopic measurement of plasma parameters and spectral line shape analysis are carried out. The essential clear difference between the NELIPS technique and NELIBS approach will be discussed.

3. Results

A summary of the basic differences between NELIPS and NELIBS is wrapped up in Table 3. It presents a succinct description of both the NELIPS technique and NELIBS approaches “without going into the fine technical details or lengthy derivations”, including the way the nanomaterials are employed, the source of enhanced emission, aims, the adopted theoretical approach to explain the enhanced emission of light, the main achievements or advantages of both approaches, challenges or limitations and the expected prospects for the future. We have therein avoided making any comparison between the NELIBS technique and other spectrochemical techniques that might employ nanomaterials.

Table 3. Differences between the NELIPS technique and NELIBS approach.

	NELIPS	NELIBS
Nature of target.	Pure nanomaterial [24,50,51,52,53,54,55,56].	A thin layer of nanomaterial deposited on the surface of the analyzed sample [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49].
2. Source of enhanced emission.	From the pure nanomaterial.	From the analyzed sample material substrate [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49].
3. Aims.	Modeling of the enhanced emission from pure nanomaterials [24,50,51,52,53,54,55,56].	Reduction in the limit of detection LOD of the LIBS–spectrochemical analytical technique [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49].
4. Recommended theory.	Thermodynamics and plasma spectroscopy.	Foundations of electromagnetic theory and plasma spectroscopy [25,32,37,38,42,43].
5. Suggested approach.	Experimentally, the OES technique was employed, followed by the careful handling of spectral data under a variety of experimental conditions conducted in a rigorous manner, including laser fluence, laser wavelength and different types of pure nanomaterials and delay times, which led to important outcomes (next item #6). Theoretically, it suggests a balance between the incident laser fluence and the plasma ignition, within the framework of the variety of the experimental findings. The thermal modeling suggests a strong reduction in the thermal conduction length of the nanomaterial to the limit of the nanoparticle diameter [52,53,54,55,56] and the validity of the law of the conservation of energy [53].	Suggested a resonance between the localized surface plasmons (LSPR) and the frequency of the incident laser light, which enhanced the coupling of laser energy to substrate materials [25,32,37,38,42,43].
6. Achievements.	Enhanced emission from plasma induced by the interaction of pulsed lasers with different pure nanomaterials was recognized as a real phenomenon inherent to nanomaterials [24,50,51,52,53,54,55,56]. The amount of enhancement rapidly declines in an exponential manner with laser fluence [24,51,52]. Enhanced plasma emission from pure nanomaterials was found to be larger at UV laser irradiation, moderate at VIS and relatively small at IR [52,56]. The analysis of the variation in the signal-to-noise (S/N) ratio from pure nanomaterials in comparison to the corresponding bulk counterpart confirmed that the plasma ignition threshold from pure nanomaterials is much lower than that from the bulk counterpart $\left({\phi }_{Th}^{Nano}/{\phi }_{Th}^{Bulk}\ll 1\right)$ [52,56]. Practically, it was identified that the reduction ratio of the threshold $\left({\phi }_{Th}^{Nano}/{\phi }_{Th}^{Bulk}\right)$ agrees with the ratio of the nanoparticle diameter to the theoretically calculated thermal conduction length of the bulk counterpart $\left({\phi }_{Th}^{Nano}/{\phi }_{Th}^{Bulk}\right)\approx \left(D^{Nano}/{\mathcal{l}}_T^{Bulk}\right)$ [52,53,54,55,56]. The plasma ignition thresholds from the pure nanomaterials $\left({\phi }_{Th}^{Nano}\right)$ were found to depend on several experimental parameters including the following: $\mathrm{The} \mathrm{inverse} \mathrm{square} \mathrm{of} \mathrm{laser} \mathrm{irradiation} \mathrm{wavelength}, \left({\phi }_{Th}^{Nano}\&{\phi }_{Th}^{Bulk}\right)\propto {\lambda }_{Laser}^{-2}$ [52,56], (using silver and zinc on forms of bulk and nanomaterial powder as a two case studies). $\mathrm{The} \mathrm{diameter} \mathrm{of} \mathrm{nanoparticles} \left(D^{Nano}\right)$ [53]. The type of nonmaterial [24,52,56], 7. The amount of enhanced emission was found: Independent of the plasma parameters $\left(n_e,T_e\right)$ [24,50,51,52,53,54,55,56], while depends on the relative population density of the ground states, $En{h}_{\lambda }=\left({\displaystyle \frac{I^{Nano}}{I^{Bulk}}}\right)\approx \left({\displaystyle \frac{N_0^{Nano}}{N_0^{Bulk}}}\right)\approx \left({\displaystyle \frac{{\rho }^{Nano}}{{\rho }^{Bulk}}}\right)\ge 1$ [24,52,53,54,55,56], as well as the type of the nanomaterial [24,52]. 8. Theoretically, the dependence of the plasma ignition threshold from the bulk and pure nanomaterials on the various parameters was explicitly derived by adopting the principal of thermal balance [52,53,54,55,56]. This model precisely predicts the measured ignition thresholds from bulk materials as well as the used pure nanomaterials, e.g., nano-zinc, as shown in Figure 1, (as a case study) [52], and with the using nano-silver material, as shown in Figure 2 [56], as well as the dependence of ignition thresholds on the inverse square laser wavelength, (using silver as a case study) as shown in Figure 3. Moreover, the model accurately predicts the dependence of thresholds on the type of material and the diameter of nanoparticles as indicated by the thick arrows in Figure 1 and Figure 2, [52,53,54,55,56].	Improvement to in the limit of detection LOD of the LIBS technique down to the range of ppb (part per billion) of the variety of the examined substrate materials [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49]. A theoretical derivation of the enhancement in the applied laser electric field based on EM theory in conjunction with the theory of surface plasmon resonance is given in refs. [37,38,42]. The introduction of the preliminary idea of the strong coupling of laser energy to substrate materials via deposited nanoparticle layers was brilliant one [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48]. There is a good theoretical assumption about the generated burning centers around nanoparticles during laser irradiation, which, in turn, would lead to a re-distribution of the incident laser light flux (energy per unit area) at the conducting substrate surface [33,34,35,36,37,38,39,40,41,42,43,46,47,48].
7. Challenges.	The unavailability of the basic thermal quantities at the nanoscale, e.g., enthalpy, coefficient of thermal conductivity, specific heat, etc. Spectroscopic challenges started to appear in a variety of recorded spectra from nanomaterials, e.g., self-reversal to some resonance lines, self-absorption to a large number of lines, absence of corrected values of transition probability coefficients of certain lines, Stark broadening parameters of other lines, etc. as given in Appendix A. The unemployment (inconsideration) of the EM theory in conjunction with localized surface plasmon resonance in the process of enhanced emission. The probable effect of the distortion in shape or distances between the nanoparticles “sintering” on amount of enhancement upon using the compression technique to put the nanoparticle powders in tablet form. Applications of NELIPS outcomes in the real-world realm are presented in Appendix A and Appendix B. Finally, so far, there is no experimental endeavor to utilize the strong continuum emission from both plasmas.	There is an excessive use of optimization parameters to reach the maximum enhancement in emission from the substrate material (e.g., the arbitrary chosen delay and gate times, laser wavelength, laser energy, thickness of the nanomaterial layer, different concentrations, different separation distances between the nanoparticles, which poses serious difficulties on the reproducibility of results [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49]. There is a need for a regular theoretical approach beyond that given in refs. [37,38,42,43]. The inconsideration of the theory of thermal processes to explain the enhanced emission from substrates; see, for example, ref. [39]. The effect of conical-shaped laser focusing in conjunction with the polarization states of the laser light, red-shifts in the resonance frequency (or absorption wavelength) of the nanoparticles during the irradiation process and nanoparticle fragmentation have not been elaborated in this one-dimensional analysis [37,38,42,43]. Eventually, the theory of enhancement in the total electric field intensity at the substrate sample appears elegant but not flexible to explain all the special cases at different conditions.
8. Expected prospects.	The principle of the invariance in the fundamental physical thermal constants at very short nanometer scales should be revisited utilizing the first to its type outcomes of NELIPS, as given in Appendix B. The role of LSPR will be considered in the process of enhanced emission from pure nanomaterials in conjunction with the thermal processes in a hybrid model capable of explaining the results of NELIBS and NELIPS as well.	Extra-fine micro-analytical chemistry promotes the potential use of the LIBS technique in a wide variety of biological, industrial and material science applications [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49].

Table 3. Differences between the NELIPS technique and NELIBS approach.

	NELIPS	NELIBS
Nature of target.	Pure nanomaterial [24,50,51,52,53,54,55,56].	A thin layer of nanomaterial deposited on the surface of the analyzed sample [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49].
2. Source of enhanced emission.	From the pure nanomaterial.	From the analyzed sample material substrate [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49].
3. Aims.	Modeling of the enhanced emission from pure nanomaterials [24,50,51,52,53,54,55,56].	Reduction in the limit of detection LOD of the LIBS–spectrochemical analytical technique [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49].
4. Recommended theory.	Thermodynamics and plasma spectroscopy.	Foundations of electromagnetic theory and plasma spectroscopy [25,32,37,38,42,43].
5. Suggested approach.	Experimentally, the OES technique was employed, followed by the careful handling of spectral data under a variety of experimental conditions conducted in a rigorous manner, including laser fluence, laser wavelength and different types of pure nanomaterials and delay times, which led to important outcomes (next item #6). Theoretically, it suggests a balance between the incident laser fluence and the plasma ignition, within the framework of the variety of the experimental findings. The thermal modeling suggests a strong reduction in the thermal conduction length of the nanomaterial to the limit of the nanoparticle diameter [52,53,54,55,56] and the validity of the law of the conservation of energy [53].	Suggested a resonance between the localized surface plasmons (LSPR) and the frequency of the incident laser light, which enhanced the coupling of laser energy to substrate materials [25,32,37,38,42,43].
6. Achievements.	Enhanced emission from plasma induced by the interaction of pulsed lasers with different pure nanomaterials was recognized as a real phenomenon inherent to nanomaterials [24,50,51,52,53,54,55,56]. The amount of enhancement rapidly declines in an exponential manner with laser fluence [24,51,52]. Enhanced plasma emission from pure nanomaterials was found to be larger at UV laser irradiation, moderate at VIS and relatively small at IR [52,56]. The analysis of the variation in the signal-to-noise (S/N) ratio from pure nanomaterials in comparison to the corresponding bulk counterpart confirmed that the plasma ignition threshold from pure nanomaterials is much lower than that from the bulk counterpart $\left({\phi }_{Th}^{Nano}/{\phi }_{Th}^{Bulk}\ll 1\right)$ [52,56]. Practically, it was identified that the reduction ratio of the threshold $\left({\phi }_{Th}^{Nano}/{\phi }_{Th}^{Bulk}\right)$ agrees with the ratio of the nanoparticle diameter to the theoretically calculated thermal conduction length of the bulk counterpart $\left({\phi }_{Th}^{Nano}/{\phi }_{Th}^{Bulk}\right)\approx \left(D^{Nano}/{\mathcal{l}}_T^{Bulk}\right)$ [52,53,54,55,56]. The plasma ignition thresholds from the pure nanomaterials $\left({\phi }_{Th}^{Nano}\right)$ were found to depend on several experimental parameters including the following: $\mathrm{The} \mathrm{inverse} \mathrm{square} \mathrm{of} \mathrm{laser} \mathrm{irradiation} \mathrm{wavelength}, \left({\phi }_{Th}^{Nano}\&{\phi }_{Th}^{Bulk}\right)\propto {\lambda }_{Laser}^{-2}$ [52,56], (using silver and zinc on forms of bulk and nanomaterial powder as a two case studies). $\mathrm{The} \mathrm{diameter} \mathrm{of} \mathrm{nanoparticles} \left(D^{Nano}\right)$ [53]. The type of nonmaterial [24,52,56], 7. The amount of enhanced emission was found: Independent of the plasma parameters $\left(n_e,T_e\right)$ [24,50,51,52,53,54,55,56], while depends on the relative population density of the ground states, $En{h}_{\lambda }=\left({\displaystyle \frac{I^{Nano}}{I^{Bulk}}}\right)\approx \left({\displaystyle \frac{N_0^{Nano}}{N_0^{Bulk}}}\right)\approx \left({\displaystyle \frac{{\rho }^{Nano}}{{\rho }^{Bulk}}}\right)\ge 1$ [24,52,53,54,55,56], as well as the type of the nanomaterial [24,52]. 8. Theoretically, the dependence of the plasma ignition threshold from the bulk and pure nanomaterials on the various parameters was explicitly derived by adopting the principal of thermal balance [52,53,54,55,56]. This model precisely predicts the measured ignition thresholds from bulk materials as well as the used pure nanomaterials, e.g., nano-zinc, as shown in Figure 1, (as a case study) [52], and with the using nano-silver material, as shown in Figure 2 [56], as well as the dependence of ignition thresholds on the inverse square laser wavelength, (using silver as a case study) as shown in Figure 3. Moreover, the model accurately predicts the dependence of thresholds on the type of material and the diameter of nanoparticles as indicated by the thick arrows in Figure 1 and Figure 2, [52,53,54,55,56].	Improvement to in the limit of detection LOD of the LIBS technique down to the range of ppb (part per billion) of the variety of the examined substrate materials [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49]. A theoretical derivation of the enhancement in the applied laser electric field based on EM theory in conjunction with the theory of surface plasmon resonance is given in refs. [37,38,42]. The introduction of the preliminary idea of the strong coupling of laser energy to substrate materials via deposited nanoparticle layers was brilliant one [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48]. There is a good theoretical assumption about the generated burning centers around nanoparticles during laser irradiation, which, in turn, would lead to a re-distribution of the incident laser light flux (energy per unit area) at the conducting substrate surface [33,34,35,36,37,38,39,40,41,42,43,46,47,48].
7. Challenges.	The unavailability of the basic thermal quantities at the nanoscale, e.g., enthalpy, coefficient of thermal conductivity, specific heat, etc. Spectroscopic challenges started to appear in a variety of recorded spectra from nanomaterials, e.g., self-reversal to some resonance lines, self-absorption to a large number of lines, absence of corrected values of transition probability coefficients of certain lines, Stark broadening parameters of other lines, etc. as given in Appendix A. The unemployment (inconsideration) of the EM theory in conjunction with localized surface plasmon resonance in the process of enhanced emission. The probable effect of the distortion in shape or distances between the nanoparticles “sintering” on amount of enhancement upon using the compression technique to put the nanoparticle powders in tablet form. Applications of NELIPS outcomes in the real-world realm are presented in Appendix A and Appendix B. Finally, so far, there is no experimental endeavor to utilize the strong continuum emission from both plasmas.	There is an excessive use of optimization parameters to reach the maximum enhancement in emission from the substrate material (e.g., the arbitrary chosen delay and gate times, laser wavelength, laser energy, thickness of the nanomaterial layer, different concentrations, different separation distances between the nanoparticles, which poses serious difficulties on the reproducibility of results [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49]. There is a need for a regular theoretical approach beyond that given in refs. [37,38,42,43]. The inconsideration of the theory of thermal processes to explain the enhanced emission from substrates; see, for example, ref. [39]. The effect of conical-shaped laser focusing in conjunction with the polarization states of the laser light, red-shifts in the resonance frequency (or absorption wavelength) of the nanoparticles during the irradiation process and nanoparticle fragmentation have not been elaborated in this one-dimensional analysis [37,38,42,43]. Eventually, the theory of enhancement in the total electric field intensity at the substrate sample appears elegant but not flexible to explain all the special cases at different conditions.
8. Expected prospects.	The principle of the invariance in the fundamental physical thermal constants at very short nanometer scales should be revisited utilizing the first to its type outcomes of NELIPS, as given in Appendix B. The role of LSPR will be considered in the process of enhanced emission from pure nanomaterials in conjunction with the thermal processes in a hybrid model capable of explaining the results of NELIBS and NELIPS as well.	Extra-fine micro-analytical chemistry promotes the potential use of the LIBS technique in a wide variety of biological, industrial and material science applications [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49].

Figure 1. The S/N ratio from Nano-ZnO-(30 nm) using Zn I line at 481nm (upper squares) and ZnO bulk (lower circles) at three laser wavelengths 1064 nm (a), 532 nm (b), 355 nm (c) (as taken from [52]). Arrows indicates different thresholds from Nano and bulk ZnO (black arrows).

Figure 1. The S/N ratio from Nano-ZnO-(30 nm) using Zn I line at 481nm (upper squares) and ZnO bulk (lower circles) at three laser wavelengths 1064 nm (a), 532 nm (b), 355 nm (c) (as taken from [52]). Arrows indicates different thresholds from Nano and bulk ZnO (black arrows).

Figure 2. The S/3N ratio from Nano-silver –(90 nm) (upper disks) and bulk (lower disks) (a) 532 nm (b) 355 nm (c) (as taken from [56]). Coloured arrows indicates different thresholds from Nano-silver and from bulk-silver (black arrows).

Figure 2. The S/3N ratio from Nano-silver –(90 nm) (upper disks) and bulk (lower disks) (a) 532 nm (b) 355 nm (c) (as taken from [56]). Coloured arrows indicates different thresholds from Nano-silver and from bulk-silver (black arrows).

Figure 3. The variation of the threshold from the nano-silver (lower squares) and from the bulk silver-based target (upper disks) at different laser wavelengths (red for 1064 nm, green for 532 nm, and blue for 355 nm), (as taken from refs [56]).

Figure 3. The variation of the threshold from the nano-silver (lower squares) and from the bulk silver-based target (upper disks) at different laser wavelengths (red for 1064 nm, green for 532 nm, and blue for 355 nm), (as taken from refs [56]).

4. Discussion

In a holistic approach to resolving the apparent overlapping between the different terms or acronyms NELIBS and NELIPS, relevant aspects are comparatively aligned in Table 3.

Regarding the first two items, both terms have employed nanomaterials in the process of laser induced plasma. In NELIPS, the target is made of pure nanomaterials [24,50,51,52,53,54,55,56], while in NELIBS, a very thin layer of nanomaterials (prepared as a dissolved matter in solution to prevent nanoparticles aggregation) are carefully deposited (by using of the simple traditional chemical equipment’s) on the surface of the examined sample material, then the solution is evaporated to get “nearly” dray surface of the tested substrate sample (metallic and non-metallic materials samples were used as in refs. [25,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49]), plant leaves as in ref. [ 32], or in unspecified methods as in refs [40,41,44,45,46,49].

In item #2, NELIBS emphasizes the enhanced emission of spectral lines is originated from the examined substrate material [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49] and rarely from the deposited nanomaterial layer [25,45]. This is in contrast to NELIPS, where the enhanced emission was found originated from the pure nanomaterial itself, which attest that, enhanced emission in NELIPS appears as an inherent to nanomaterials [24,50,51,52,53,54,55,56].

In item #3, NELIBS candidates pursue an improvement in the limit of detection (LOD) of the LIBS technique via an enhancement in the spectral line intensities that emerged from the minor concentration elements in the tested sample matrix [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49]. While, the NELIPS aims at modeling the physical reasons and the mechanism of enhanced emission originated from plasmas produced by the laser interaction with that class of materials [24,50,51,52,53,54,55,56], in particular, at different levels of the used laser flux (energy per unit area) and at different laser irradiation wavelengths with different types of nanomaterials having different diameters [53], but in a regular and very systematic manner.

In item #4, in order to explain theoretically the observed spectral line enhancement, there are actually two viable ways in both NELIPS and NELIBS, based on two different points of view. In NELIPS, the thermal processes (thermodynamics) in conjunction with optical emission spectroscopy (OES) measurements were taken seriously into consideration since pulsed laser induced plasmas from any material is a photo-thermal process [2,3,4,5,6,7,8,9,10,11]. In NELIBS, the electromagnetic theory of surface plasmon resonance with incident laser light frequency was claimed to explain the results of enhanced emission from the sample substrates layered with nanomaterial [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49].

Item #5 recalls ideas about the suggested approach to explain experimental findings by both NELIPS and NELIBS.

Concerning NELIBS processing, basically, it is assumed that the incident electromagnetic laser light induces a resonance excitation of surface plasmons between the added nanoparticles. Consequently, this increases the strength of the localized electric field (between the nanoparticles and the substrate) by about two up to four orders of magnitude [25,37,38,42]. This localized strong-field points act like a set of ignition centers [33,34,35,36,37,42] at which the magnitude of the laser irradiance jumps by four up to nine orders of magnitude ($\because I_{Laser}\propto {\left|E_{Total}\right|}^2$). This staggering level of laser irradiance around the figure of $I_{Laser}\approx {10}^{14-19}\left(\mathrm{Watt}.{\mathrm{cm}}^{-2}\right)$ enables the strong field ionization process to take place (quantum tunneling effect) [33,37,38,42]. This tunneling process liberates a tremendous number of electrons during the first few cycles of the laser pulse (less than one femtosecond) [40], producing localized heat centers at the substrate surface. This would subsequently start plasma ignition via the traditional inverse-bremsstrahlung process [33,37,38,42]. This sophisticated theory suggests a very efficient process of laser induced breakdown [33,37,38,42] and, hence, the creation of extremely luminous plasma characterized by strong spectral lines originating from the substrate material rather than the added nanomaterial. A semi-classical approach to estimate the total electric field intensity induced between the added nanoparticles was carried out in refs. [37,38,42,43,46,47]. It predicts immense enhancement in the localized electric fields only if the inter-nanoparticle distances become very small compared with their radii. Practically, the increased concentration of nanoparticles on the surface of tested substrate materials turns out to be a good idea [37,38,42,43,45,46,47].

Nevertheless, in ref. [41], NELIBS of liquid samples was reported by using modified surface-Enhanced Raman Scattering Substrates (ERSSs) without presenting a regular theoretical model. In ref. [44], a layer of copper nanoparticles of an average diameter of 153 nm was deposited on an aluminum substrate with a reported increase in enhanced emission by a factor of about 50X, and the enhanced emission was attributed “as usual” to the effect of LSPR. Moreover, in ref. [45], three different types of nanoparticles of copper, magnesium and gold were deposited as thin layers to the surface of an aluminum substrate, which resulted in relatively small enhancement factors of 5.3, 2.2 and 2.2, respectively, from the aluminum substrate. There was also an observed enhanced emission from spectral lines recognized from an unknown element that was not included in the substrate material matrix (sodium), as well as a significant rise in electron temperature without any appreciable increase in electron density. The observed spectral line enhancement, in addition to any other changes in the measured plasma parameters, was attributed to the effects of LSPR and the concentration of the deposited nanomaterial layer on substrates or, in other cases, to the applied external magnetic field [40,44,45,46]. Moreover, in ref. [49], the NETag-LIBS technique was suggested to improve the LOD in NELIBS after very sophisticated procedures.

However, a short, remarkable pilot study was presented in ref. [39]. The thermal modeling approach was proposed to explain the observed enhancement in spectral lines that emerged from the Ti substrate sample covered with a thin layer of 20 nm gold nanoparticles.

On the other hand, in NELIPS, the well-established theory of thermodynamics was considered adequate to corroborate the assumptions of modeling [52,56]. Laser induced plasma emissions of the same materials (bulk and nanomaterial of similar stoichiometry) have been investigated using the OES technique [24,50,51,52,53,54,55,56]. Under similar experimental conditions, two spectra pertaining to bulk material and its nanomaterial counterpart were acquired. Next, the corresponding signal-to-noise (S/N) ratio for both spectra was then drawn as a function of incident laser fluence, as shown in Figure 1 and Figure 2 [52,56]. Backward extrapolation was traced on the linear low-fluence part of the S/N ratio to intersect the horizontal laser fluence axis at a point at which the international standard of S/N = 3 [52,56]. The points of intersection are identified as the plasma ignition threshold $\left({\phi }_{Th}^{Nano},{\phi }_{Th}^{Bulk}\right)$. Figure 1 indicates that not only do nanomaterials exhibit higher signal-to-noise ratios, but they are also susceptible to lower ignition thresholds $\left({\phi }_{Th}^{Nano}\ll {\phi }_{Th}^{Bulk}\right)$ [52,56]. Moreover, different plasma ignition values were obtained in the case of incident laser beams of shorter wavelengths, as shown in Figure 1 and Figure 2, as taken from [52,56]. Furthermore, a simple relation could be extracted, as shown in Figure 3, where the plasma thermal ignition threshold of the pure nanomaterial, as well as the bulk counterpart, correlate with the inverse square of the laser excitation wavelength $\left({\phi }_{Th}^{Nano},{\phi }_{Th}^{Bulk}\right)\propto {\lambda }_{Laser}^{-2}$ [52,56]. Moreover, as shown in Figure 4, there is an exponential decrease in the amount of enhanced emission from pure nanomaterial with incident laser flux, which can be explained in terms of the previous Figure 1 and Figure 2 via simple mathematical ratio as following $Enh.={\displaystyle \frac{{\left(S/N\right)}^{Nano}}{{\left(S/N\right)}^{Bulk}}}$ [52,53,56].

A plausible observation could be maintained to relate the ratio of the reduction in the plasma ignition threshold of nanomaterials with respect to its bulk counterpart to the ratio of nanoparticle diameter $D^{Nano}$ to the theoretically calculated thermal conduction length of the bulk material ${\mathcal{l}}_T^{Bulk}=\sqrt{k_T^{Bulk}{\tau }_L/{\rho }^{Bulk}{C}_P^{Bulk}}$ so that

$$\left({\phi }_{Th}^{Nano}/{\phi }_{Th}^{Bulk}\simeq D^{Nano}/{\mathcal{l}}_T^{Bulk}\right)\ll 1$$

(2)

where $k_T^{Bulk}$ denotes the coefficient of thermal conductivity, ${\rho }^{Bulk}$ density, $C_P^{Bulk}$ isochoric heat capacity, and ${\tau }_L$ laser pulse duration.

Nevertheless, it was a good idea to explore the ZnO nanomaterials plasma ignition thresholds as a function of nanoparticle diameter (diameters of 20, 40, 70 and 100 nm were employed in the experiment in ref. [53]) under similar experimental conditions. Surprisingly, the nanomaterial plasma ignition threshold was found to decrease in a linear manner with the nanomaterial diameter $\left({\phi }_{Th}^{Nano}\propto D^{Nano}\right)$.

For comparison reasons, the traditional measurement was carried out using plasma parameters of both plasmas that originated from bulk and pure nanomaterials $n_e^{Nano},n_e^{Bulk};T_e^{Nano},T_e^{Bulk};N_0^{Nano},N_0^{Bulk}$ [50,51,52,53,54,55,56], after corrections against the effect of self-absorption (with details as given in refs. [24,52,56]). This would imply that $n_e^{Nano}\simeq n_e^{Bulk};T_e^{Nano}\simeq T_e^{Bulk}$, but it was found that $N_0^{Nano}\gg N_0^{Bulk}$ whereby pure nanomaterial enhanced emission is independent of the relative electron density and electron temperature [24,52,56] but directly proportional to the relative population density of the ground states (relative density or relative concentration), i.e., one can write the following practical relation [24,51,52,53,54,55,56] $En{h}_{\lambda }=\left({\displaystyle \frac{I^{Nano}}{I^{Bulk}}}\right)=={\displaystyle \frac{{\left(S/N\right)}^{Nano}}{{\left(S/N\right)}^{Bulk}}}\approx \left({\displaystyle \frac{N_0^{Nano}}{N_0^{Bulk}}}\right)\approx \left({\displaystyle \frac{{\rho }^{Nano}}{{\rho }^{Bulk}}}\right)\ge 1$.

Theoretically, according to item #5, concerning NELIPS, the principle of thermal balancing (thermodynamics) was claimed to explain the relevant experimental findings. Thereupon, a classical threshold of plasma ignition at the surface of bulk materials was adopted. However, the following expression (3) was elaborated to not only account for target material evaporation as the old theory in classical thermodynamics $\left({\phi }_{Th}^{Classic}={\rho }^{Bulk}{L}_V^{Bulk}{\mathcal{l}}_T^{Bulk}\right)$, but rather include an additional ionization term, $\left({\phi }_{Th}^{ionization}=\left({\displaystyle \frac{4{\pi }^2{m}_e{\epsilon }_o{c}^2}{e^2}}{\displaystyle \frac{{\epsilon }_i}{{\lambda }_{Laser}^2}}\right){\mathcal{l}}_T\right)$, derived explicitly in refs. [52,56]. Hence, the total plasma ignition threshold from bulk materials is given by

$${\phi }_{Th}^{Bulk}=\left({\rho }^{Bulk}{L}_V^{Bulk}+{\displaystyle \frac{4{\pi }^2{m}_e{\epsilon }_o{c}^2}{e^2}}{\displaystyle \frac{{\epsilon }_i}{{\lambda }_{Laser}^2}}\right){\mathcal{l}}_T$$

(3)

The different symbols are given in the list of symbols. To attest to this relation, a fair agreement with experimentally measured values of plasma ignition thresholds was reported [52,53,54,55,56], as shown in Figure 1 and Figure 2. Nonetheless, as a final step and in order to find a similar expression that can correctly predict the experimentally measured nanomaterial plasma ignition threshold (as shown in Figure 1 and Figure 2),${ \phi }_{Th}^{Nano}$, we make use of Equation (3) to obtain

$${\phi }_{Th}^{Nano}=\left({\rho }^{Nano}{L}_V^{Nano}+{\displaystyle \frac{4{\pi }^2{m}_e{\epsilon }_o{c}^2}{e^2}}{\displaystyle \frac{{\epsilon }_i}{{\lambda }_{Laser}^2}}\right)D^{Nano}$$

(4)

Here, according to the law of conservation of energy, it was assumed that the thermal conduction length of the nanomaterials should be reduced to the limit of the diameter of the nanoparticles ${\mathcal{l}}_T^{Nano}\to D^{Nano}$. A theoretical derivation based on the theory of thermal balancing, as well as experimental data corroborating this equation, has been reported [52,53,56].

Concerning item #6, part of the experimental findings, as well as the theoretical point of view, was reported for the first time in previous publications [24,52,56]; therefore, these were considered as an achievement for NELIPS.

In this regard, there was a remarkable improvement in the limit of detection LOD of the LIBS technique by NELIBS down to the range of ppb (part per billion) [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49].

Concerning item #7, in NELIPS, the lack of available reliable data about the thermal parameters of the different types of nanomaterials constitutes a real challenge. This might stem from experimental difficulties and/or the scarcity of funds allocated to similar projects. This issue has been circumvented in favor of the general validity of the concept of conservation of energy, i.e., it was assumed the validity of the relation [52] $\left({\rho }^{Bulk}{L}_V^{Bulk}\right)=\left({\rho }^{Nano}{L}_V^{Nano}\right)=\left(\rho L_V\right)=const.$ (as can be calculated from the slope of the linear relation between threshold of plasma ignition and the diameter of nanoparticles [53]) regardless of the class of the used material [53]. Meanwhile, this, fortunately, helps establish a theoretical relation regarding the nanomaterial plasma ignition threshold of zinc and silver nanomaterials, as was successfully applied in the case studies in refs. [52,56]. Furthermore, the probable distortion of the nanoparticle shapes or inter-nanoparticle distance by the sintering effect during target preparation by the compression of nanoparticle powder into tablet form should be monitored as well. After all, the authors urged for applying the basic EM theory to explain enhanced emission from pure nanomaterials, but in a more rigorous way.

However, certain limitations referred to in item #7 for the NELIBS approach have to do with more efforts to establish more regular measurement procedures to readily reproduce optimal enhancement levels (without changing all the experimental parameters at the same time in order to obtain the highest level of enhanced emission from the substrate material). In addition, some technical problems concerning the physical nature of the spectral shift in the peak absorption wavelength of nanoparticles as a function of the inter-nanoparticle distances have not been taken into consideration in the theoretical modeling [37,38,42]. There are possible, obvious problems concerning the wetting of both nanoparticles and the examined substrates by the solution containing the nanomaterial, which might lead to unfortunate predictions, as observed in refs. [44,45]. Furthermore, the theory of localized surface plasmon resonance disregards the analysis of parallel and vertical components (TE or TM polarization states) of the incident oscillating laser electric field in a conical-shaped laser due to the laser focusing lens at the interaction spot. Another refinement of the suggested EM theory in conjunction with the theory of LSPR would be to directly relate the amount of enhanced light emission from plasmas that emerged from substrates to the surface plasmon resonance parameters. These parameters should include the resonance frequency of the nanoparticles, size of the nanoparticles and inter-nanoparticle distances, as well as the suitable laser irradiance level, the laser wavelength or frequency in comparison to the plasmon resonance frequency, the laser pulse duration and the angle of incidence of laser light on the target, taking into consideration the two states of the polarization of the incident laser light. In this context, from the physics point of view, the assumed role of the sudden tunneling of a large stream of electrons from substrate materials (during the first few laser cycles, ~1 femtosecond [37,38,42,43]) means that as the incident laser irradiance increases, the number of ejected electrons would increase as well. Consequently, one should obtain a higher amount of enhanced emission from substrates as the laser irradiance increases. This result is in clear contradiction to the decreasing trend of enhancement with the increase in laser irradiance. Moreover, spectroscopically, there is large confusion about the general trend of the measured plasma parameters [44,45] (electron density and temperature), especially under the assumption about the sudden creation of a large stream of tunneling electrons as a result of the effect of LSPR [37,38,42]; consequently, and at least, one should observe a sudden increase in the measured plasma electron density or a strong continuum emission at the early delay times. This is in clear contradiction to the experimentally verified constant levels of electron density regimes without any increase in the continuum emission. However, there is a good level of confidence about the measured linear proportionality of enhanced emission with an increased concentration of nanoparticles [25,37,38,42]; this is without too much information about the homogeneity or the thickness of the deposited nanomaterial layers, especially when using the popular deposition methods.

Ultimately, the practical empirical findings are brilliantly far-reaching but, unfortunately, without a reliable rigorous sophistication theory and need critical revision by specialists in the region of theoretical solid-state physics.

However, as indicated in item #8, the NELIBS technique is well-established and opens the door to the extra-fine, simple and cheap useful micro-analytical technique, promoting the potential use of LIBS in a wide range of biological, industrial and material science applications [25,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49].

On the other hand, in item #8, NELIPS will have to undertake efforts to build up a hybrid theoretical model that includes both thermal and EM theories to explain the observed nanomaterial plasma emission enhancement. Eventually, one should look forward to conceiving simple experiments to measure the variation in the thermal constants at the nanometer scale. Meanwhile, the first of this genre of outcome for NELIPS concerning the two quantities (${\phi }_{Th}^{Bulk\&Nano}$) and the verified assumption about the reduction in the thermal conduction length of nanomaterials to the limit of the diameter of nanoparticles should be incorporated in a more refined thermodynamic approach. The principle of the invariance in the fundamental physical thermal constants at very short nanometer scales should be revisited as well. The challenges in the OES technique and applications of NELIPS outcomes in the real world will be discussed in Appendix A.

Finally, we discuss the possible scenarios and suggestions to conduct different experiments to explore the degree of stability or variation in the basic thermal quantities at the nanometer scale. A hint about that is presented in the annexed Appendix B. This may set off the motivation to start to investigate these constants using the NELIPS method, and probably, this may provide the main achievement of NELIPS for the future.

5. Conclusions

The interaction of pulsed lasers with matter involving nanomaterials as a pure target or thin layer deposited on a target initiates transient plasma, which shows a strong enhancement in spectral line emission. This domain of research has been explored via two well-established techniques dubbed NELIBS and NELIPS, entailing similarities as well as differences. A subtle differentiation between these two techniques was clearly presented, depending on the nature of the target, the origin of enhancement and the prevalent theoretical approaches. The potential achievements, challenges and expected prospects were highlighted as well. While we emphasized NELIPS as a powerful tool using the optical emission spectroscopy (OES) technique, we call for further theoretical and experimental exploration of the degree of stability of the fundamental thermodynamic constants at the nanometer scale, without any bias to preliminary ideas. These two approaches must basically be complementary, but honestly, further collaborative efforts should be carried out to look for common challenges. This would pave the way for a more rigorously robust theory of the interaction of pulsed lasers with nanomaterials as well as establish more regular measurements of the basic thermal constants at the nanometer scale.