Laser induced breakdown spectroscopy of aluminum incorporated with metallic nanoparticles

Laser-Induced Breakdown Spectroscopy is a promising spectroscopic technique with a vast spectrum of applications in fields concerned with identification and detection of elements. But it faces some limitations due to self-absorption, noise due to matrix effect and line broadening resulting in low emission signal. This research proposes LIBS signal enhancement by incorporation of metal nanoparticles (Cu, Mg, Au) on Al surface and compares their effect. The successful optical emissions enhancement is achieved as the emission intensities of Al- and Na- lines of three coated samples are compared with those of uncoated Al. The Electron Temperature has been evaluated by Boltzmann plot and an increase in Electron Temperature has been observed with the incorporation of nanoparticles to the aluminum surface as compared to the untreated aluminum, due to more plasma emissions. The Electron Number Density of the aluminum plasma did not have much effect with the incorporation of Nanoparticles. The Local Thermal Equilibrium condition has been satisfied and checked by Mc Whirter’s Criterion. The incorporation of metal nanoparticles can be declared as an effective method not only for LIBS signal enhancement but also better detection of trace elements which were not observed without the use of Nanoparticles.


Introduction
Laser Induced Breakdown Spectroscopy (LIBS) is a technique in which plasma is generated by a high-power laser beams and light is emitted as this plasma cools down (Jun and Kim 2018). LIBS utilizes the atomic emissions of elements through its spectroscopy which provides fast, multi-element analysis of different materials in all physical states of matter i.e. gas, liquid or solid. Its main advantage is that its instrumentation is comparatively simple and robust that is also capable of providing fast in situ analyses of materials with very less or no need of former preparation of samples at all (Harmon et al. 2018). The application of LIBS analysis spreads in many fields such as archaeology (Singh et al. 2018), geology (Korala et al. 2018), agriculture (Peng et al. 2016), food industry (Sezer et al. 2018) and criminology (Doña-Fernández et al. 2018) etc. Due to its vast applications in many fields, improvement in the sensitivity of this technique has become the main concern. For this purpose, many ideas and techniques have been proposed and experimented, among which are the multiple-pulse LIBS (Babushok et al. 2006;Giacomo et al. 2008), application of magnetic and electric fields during LIBS (Harilal et al. 2004;Robledo-Martinez 2018), Resonance LIBS (Goueguel et al. 2010) and LIBS under vacuum conditions (Sladkova et al. 2017). All these techniques have been reported to enhance the LIBS signal by their respective phenomenon but the only discrepancy is that the simple LIBS setup becomes complex and expensive. This discrepancy has been removed by a recently proposed enhancement technique which proposes the incorporation of nanoparticles on the sample surface and does not involve massive changes in conventional experimental setup of LIBS. This technique was named as Nanoparticle Enhanced Laser Induced Breakdown Spectroscopy (NELIBS) which improves the sensitivity of LIBS by increasing the deposition of laser energy from laser to sample (Giacomo et al. 2014) without changing the chemical properties of sample. Furthermore, the modification in experimental setup is not complex. Only a coating of nanoparticles is to be applied on the sample surface which is removed during the laser-matter interaction and the sample is not contaminated. So far, NELIBS has been done by using Silver (Ag) nanoparticles for the enhanced molecularband detection (Koral et al. 2016), Gold (Au) nanoparticles for the enhanced LIBS spectra of precious gem stones by inducing less damage due to presence of nanoparticles (Korala et al. 2018), Silver (Ag) nanoparticles along with vacuum conditions where it was concluded that vacuum conditions had no advantage over ambient conditions (Sladkova et al. 2017), Gold (Au) nanoparticles combined with magnetic field and an optimum energy of 80 mJ was found best for enhancement using the combination of nanoparticles and magnetic field (Tang 2018). Green and bio-synthesized nanoparticles of Silver (Ag) have also been investigated for enhancement of LIBS spectra (Poggialini et al. 2018;Abdel-Salam 2018) and different sizes of Gold (Au) nanoparticles have been compared along with double-pulse LIBS and promising results have been achieved (Yang et al. 2017). Different concentrations of same nanoparticle have been compared for the best and optimized results and it was found that too much concentration of nanoparticles lowers the enhancement due to agglomeration of nanoparticles on the surface (Dell'Aglio 2018).
The aim of this work is to find out whether the incorporation of three different types of nanoparticles on aluminum substrate just only enhances the emission signals of LIBS or there is also improvement in trace element detection. From our results, it is found that the incorporation of NPs not only improved the LIBS emission signals but also better detection of trace elements which is the novelty of this work. Moreover, plasma parameters have also been calculated and compared for Al and Nanoparticles treated Al.

Experimental details
Aluminum metal was taken as sample to take LIBS Spectra in the presence and absence of three types of metal nanoparticles. The nanoparticles used were those of Copper (Cu), Magnesium (Mg) and Gold (Au). One sample of Aluminum was kept untreated and three samples were coated with these three types of nanoparticles. The nanoparticles were prepared in neon atmosphere by Pulsed Laser Ablation in Liquid (PLAL) taking Acetone as the solvent for preparation of colloidal solution of nanoparticles. Neon atmosphere prevented the danger of catching fire when laser was shot on acetone immersed sample. For this purpose, a specially designed chamber was used. 20 µL of colloidal solution of each nanoparticle was coated on three Aluminum samples by spin-coating method at a rate of 500 RPM initially which then attained 1000 RPM so that the particles are distributed evenly on the surface and excess liquid splashed out if any. This combination of rotations was applied for 1 min. A "Quantel Twins Brilliant b" Q-Switched, Flashlamp pumped, pulsed Nd:YAG double pulse laser was used for the synthesis of nanoparticles as well as to carry out Laser-Induced Breakdown Spectroscopy of the samples incorporated with nanoparticles. The laser operates at a temperature of 38 °C at a power of 8.5 W at a frequency of 10 Hz. Maximum achievable energy of the laser is 850 mJ, however the experiment was carried out at an optimized energy of 130 mJ for Nanoparticle synthesis as well as for LIBS of the samples. The laser was operated at a wavelength of 1064 nm and it has a pulse duration of 6 ns. The unfocused diameter of the laser is 1 cm which after focusing gives a focused diameter of 300-400 µm. This laser was focused on the Aluminum sample by a well-calibrated arrangement of optics (Fig. 1) and a plasma was achieved. The optical emissions from plasma were collected by an optical fiber having a diameter of 600 µm and taken to a spectrometer (LIBS2500plus spectrometers by Ocean Optics) after a delay time of 333 µs, where these optical emissions were split into individual wavelengths as a function of their angle and were fed to a detector (CCD Detector in our case) for the detection of the spectral lines associated with these wavelengths. These spectral emission lines were then plotted in a computer using OOILIBSplus which is a Windows-based analysis software. Spectral resolution of the used spectrometer is ~ 0.1 nm which is less than the stark broadened LIBS lines so the system is able to resolve every spectral line.
The uncertainties of the data were overcome by taking ten laser shots on the sample at different locations and monitoring the portion of the laser energy of each laser shot. The beam splitter was used to split 30% laser light and incident on the calibrated energy meter. Only those ten spectra were recorded which had the same energy on the energy meter. It was a sort of online laser energy monitoring. The average value of these ten spectra was used for the analysis in this work. In this way, the fluctuations and uncertainties were minimized and the data is more reproducible.

Scanning electron microscopy
Firstly, the nanoparticles of three metals are successfully prepared and their size was confirmed by Scanning Electron Microscope (SEM-MAIA3 TESCAN). Figure 2 shows the scanning electron micrographs of copper, magnesium and gold nanoparticles on Al surface. Figure 2a and b show the copper nanoparticles which are almost spherical in shape having size 10-20 nm. It can also be seen that these small nanoparticles agglomerate to form bigger particles but the size of big separated particles are still below 100 nm. Figure 2c and d) show the magnesium nanoparticles on Al surface having size 40-50 nm and Fig. 2e and f show that gold nanoparticles are also formed having size 25-50 nm. From these results, it can be seen that the size of particles formed during pulsed laser ablation of copper, magnesium and gold in liquid are below 100 nm.

LIBS measurements and analysis
After deposition of nanoparticles on Al surface, the LIBS spectra (    Fig. 3 is that the emission intensity of the aluminum samples incorporated with all three types of nanoparticles is greater than the untreated aluminum. Secondly, the more number of peaks are detected in each spectra of nanoparticles incorporated aluminum than untreated aluminum which shows the improvement in trace-element detection ( Fig. 3 and Table 1). Thirdly, the comparison of emission intensity of two elements, Al at ~ 396 nm and Na at 589 & 589.6 nm, is done in Fig. 4 that shows that the emission intensity of Al is increased when coated with nanoparticles. The emission intensity of aluminum peak at 396 nm is increased by 48% in case of Au nanoparticles, 64% in case of Cu nanoparticles and 71% in case of Mg nanoparticles (left inset of Fig. 4). The emission intensity of sodium peak at 589 nm is also increased by 100% in case of Au nanoparticles, 423% in case of Cu nanoparticles and 123% in case of Mg nanoparticles (right inset of Fig. 4). The enhancement of LIBS signal by incorporation of nanoparticles is attributed to Surface Plasmon Resonance (SPR) which is a phenomenon prominent in structures in which the surface-to-volume ratio is high as in the case of nanoparticles. The surface percentage is almost negligible in bulk materials as well as micro-particles but it is a significant part of a nanoparticle. Due to this fact, the incident laser illuminates the whole nanoparticle. Due to the very small size of nanoparticles ⁓10 -9 m, the electrons arrange themselves on the surface of the particle around a central positive core. Incident laser or electromagnetic field causes this electron density to oscillate around this positive  core, the displacement of this electron density from its mean position is called as Plasmon. This collective motion of electrons induces a dipole on the nanoparticles whose oscillation frequency is same as that of the incoming laser. This causes a resonance of the both fields and this momentary field of induced dipoles enhances the electromagnetic field of laser but this enhancement is localized on the surface of the nanoparticle only. This makes the nanoparticles hot centers of localized energy. This energy is then delivered thermally to the sample surface and very localized ablation of material is achieved inducing less damage to the sample surface. In bulk materials, when the surface plasmons absorb electromagnetic radiation, they cannot oscillate but they propagate along the surface. This is the reason that SPR is not prominent in bulk materials and this causes enhancement due to nanoparticles (Dell'Aglio 2018).

Electron temperature
The Electron Temperature was calculated using the Integrated Intensities of emission lines of a single type of element in single ionization state. As Local Thermodynamic Equilibrium (LTE) condition is satisfied in cold plasmas like in our experiments (also proved in next discussion), the distribution of level populations according to Boltzmann law gives the relative intensity in the following expression (Eq. 1): (1) = − + where λ is the wavelength of emitted radiation, I is the Integrated Intensity of the peak, g is the degeneracy, E is the Energy of upper level of transition (in eV), k is the Boltzmann's constant, A is the Transition probability (s −1 ), T is the electron or plasma temperature, h is the Planck's constant (Js), N • is the Number density of neutral atoms (m −3 ) and Q • is the partition function of neutral atoms (Gondal and Habibullah 2016). Electron Temperature for each sample is calculated by taking neutral Al (Al I) spectral lines. Figure 5 shows that the linear fitting of plotted points between ( I gA ) and E gives a slope of −1.09 for untreated Aluminum, which shows Electron Temperature of 10,646 Kelvin. Similarly, −0.88 is the slope of linear fitting for Cu nanoparticles coated Al, giving an Electron Temperature of 13,100 K. A slope of −0.96 is obtained for Aluminum coated with Mg Nanoparticles for which Electron Temperature is found to be 12,100 K. Linear fitting of Al I emission lines for Aluminum coated with Au nanoparticles having slope of -0.93 for which calculated Electron Temperature is 12,400 K. It has been observed that the Electron Temperature is higher in case of nanoparticles incorporation on aluminum surface as compared to

Electron number density
The electron number density of a plasma can be found out by the broadening caused due to the fast-moving electrons interaction with neutral atoms or ions. The full width at half maximum (FWHM) of Stark-Broadened spectral lines was used to calculate the Electron Number Density of the plasma. Hydrogen Alpha (H α ) line at 656.28 nm was used to calculate the Electron Number Density using Eq. 2: where, α 1∕2 is the Half Impact Parameter or Stark Broadening Parameter, whose value is taken from literature corresponding to the range of Electron Temperature of plasma and is different for different ranges of electron temperature. Figure 6 shows the Hydrogen Alpha lines of all four samples through which the FWHM values are determined for the calculation of electron number density. The electron number density (N e ) is found nearly equal (2) N e = 8.02 × 10 12 Δ 1∕2 α 1∕2 for all samples i.e. around 3 × 10 15 cm −3 . Electron number densities (N e ) should be significantly large for LTE condition to prevail because LTE is said to exist when the major cause of de-excitation is electron collisions and that is only possible when the electrons are so many in quantity that de-excitation due to collisions exceeds any other possible mechanisms of de-excitation (Yaseen Iraqi 1584). The electron number density (N e ~ 10 15 ) is found to be greater than McWhirter's factor (~ 10 14 ) by one order of magnitude satisfying the McWhirter's criteria (Eq. 3) for LTE condition.
The values of electron temperature, FWHM of H α line, electron number density and McWhirter's factor are tabulated in Table 2. The Electron Temperature of aluminum plasma has higher value with the induction of nanoparticles on it, while the number density of untreated and Nanoparticles treated aluminum are comparable.

Conclusions
Aluminum plasma has been generated by Nd: YAG laser (laser energy 130 mJ and wavelength 1064 nm) without and with the incorporation of copper, magnesium and gold nanoparticles. The induction of nanoparticles on an aluminum surface shows an average enhancement in LIBS signals up to 5.2 times for Copper Nanoparticles, 2.2 times for Magnesium and Gold Nanoparticles as compared to Nanoparticles free LIBS. A significant enhancement in the electron temperature with the incorporation of Nanoparticles has been observed, while a little deviation in electron number density for all samples has been found. The enhancement in emission signals and electron temperature is due to the better interaction of laser beam with the aluminum surface in the presence of nanoparticles. From our results, it can also be concluded that the incorporation of NPs not only improves the LIBS emission signals as reported earlier but also better detection of trace elements becomes possible, which is the novelty of this work. So the better performances are achieved in both cases. NELIBS has many potential applications, where the LIBS spectra is generally weak e.g. in quality-control sectors like the packaging of goods, some eatables, aerospace industry, soil and water etc.