Improving the Signal Strength and Detection Limits …
313
mJ) with enhanced emission intensity. The improved sensitivity and detection limit
achieved by using colloidal NPs, is contributed by the localized surface plasmon
(LSP). The effect of size, material, and concentration of the metallic particles on
the LSP enhanced spectral intensities were also demonstrated. However, they have
not provided extensive information about the role of the nanoparticles induced signal enhancement in LIBS. In the later stage, 2012, Sherbini et al. observed an 8-fold
increment of Zn-I emission line from a nano-based ZnO target over a bulk-based ZnO
target [35]. They ascribed the enhancement to the higher concentration of neutral zinc
atoms in the Nano-based material plasma than the Bulk-based material plasma. They
have also reported that plasma parameters such as temperature and electron density
are identical in both cases.
On the other hand, De Giacomo et al. performed detailed systematic experimental
studies and provided a satisfactory explanation of the properties of metallic nanoparticles when it interacts with high energy laser pulses and the mechanism of laser
ablation enabled by the presence of nanoparticles [14]. They argued that the nanoparticles on the surface of any bulk materials behave like an efficient thermally insulated
defect and an excellent seed electrons source. The nanoparticles’ breakdown threshold is much lower than the bulk materials; hence, the nanoparticles deliver a large
number of seed electrons that locally increase the temperature of the interaction zone.
Consequently, multiple plasma ignitions occur at the interaction zone that, in turn,
lowers the ablation threshold of the substrate and increases the ablation efficiency.
Thus, the nanoparticles induce energy deposition on the surface and subsequently
increase the light-matter interaction. However, this phenomenon will only be significant when the laser irradiance is extremely low. On the contrary, at higher irradiance
(above the breakdown threshold), multiphoton ionization dominates. Thus, the NPs
induce the electromagnetic field enhancement (few orders of magnitude) as a result
of plasmon coupling. The laser pulses interacting with the NPs cause coherent and
collective oscillations of delocalized electrons in the NPs and thus amplify the incident electromagnetic field (1–3 orders of magnitude), which increases the electric
field in the interaction zone. This induced local electric field enhancement produces
seed electrons through field emission and thus improves breakdown and plasma production efficiency [36, 37]. The more reliable explanation of the NPs enhancement
for the LIBS intensity is ascribed to the plasmonic properties of NPs, as mentioned by
multiple authors [34, 38, 39]. However, many scientific aspects need to be addressed
and the mechanism of laser ablation enabled by the nanoparticles is yet to be fully
understood.
De Giacomo reported 1–2 orders of magnitude enhancement of the LIBS signal of
metals by depositing metallic (silver) nanoparticles, whereas no substantial improvement was observed for high threshold samples such as insulators and semiconductors
[40]. The nanoparticles reduce the breakdown threshold of various metals ranging
from 19 to 35%, while no changes occur for semiconductors and insulators. They have
also demonstrated that the size (10–20 nm) and concentration (0.02–0.004 mg/mL)
of the Ag nanoparticles do not influence the metals’ emitted signal strength. The
authors also reported that NELIBS enabled detection lowers the limit of detection
(LOD) by more than 1 order of magnitude in the case of Mn and Pb in copper-based
313
mJ) with enhanced emission intensity. The improved sensitivity and detection limit
achieved by using colloidal NPs, is contributed by the localized surface plasmon
(LSP). The effect of size, material, and concentration of the metallic particles on
the LSP enhanced spectral intensities were also demonstrated. However, they have
not provided extensive information about the role of the nanoparticles induced signal enhancement in LIBS. In the later stage, 2012, Sherbini et al. observed an 8-fold
increment of Zn-I emission line from a nano-based ZnO target over a bulk-based ZnO
target [35]. They ascribed the enhancement to the higher concentration of neutral zinc
atoms in the Nano-based material plasma than the Bulk-based material plasma. They
have also reported that plasma parameters such as temperature and electron density
are identical in both cases.
On the other hand, De Giacomo et al. performed detailed systematic experimental
studies and provided a satisfactory explanation of the properties of metallic nanoparticles when it interacts with high energy laser pulses and the mechanism of laser
ablation enabled by the presence of nanoparticles [14]. They argued that the nanoparticles on the surface of any bulk materials behave like an efficient thermally insulated
defect and an excellent seed electrons source. The nanoparticles’ breakdown threshold is much lower than the bulk materials; hence, the nanoparticles deliver a large
number of seed electrons that locally increase the temperature of the interaction zone.
Consequently, multiple plasma ignitions occur at the interaction zone that, in turn,
lowers the ablation threshold of the substrate and increases the ablation efficiency.
Thus, the nanoparticles induce energy deposition on the surface and subsequently
increase the light-matter interaction. However, this phenomenon will only be significant when the laser irradiance is extremely low. On the contrary, at higher irradiance
(above the breakdown threshold), multiphoton ionization dominates. Thus, the NPs
induce the electromagnetic field enhancement (few orders of magnitude) as a result
of plasmon coupling. The laser pulses interacting with the NPs cause coherent and
collective oscillations of delocalized electrons in the NPs and thus amplify the incident electromagnetic field (1–3 orders of magnitude), which increases the electric
field in the interaction zone. This induced local electric field enhancement produces
seed electrons through field emission and thus improves breakdown and plasma production efficiency [36, 37]. The more reliable explanation of the NPs enhancement
for the LIBS intensity is ascribed to the plasmonic properties of NPs, as mentioned by
multiple authors [34, 38, 39]. However, many scientific aspects need to be addressed
and the mechanism of laser ablation enabled by the nanoparticles is yet to be fully
understood.
De Giacomo reported 1–2 orders of magnitude enhancement of the LIBS signal of
metals by depositing metallic (silver) nanoparticles, whereas no substantial improvement was observed for high threshold samples such as insulators and semiconductors
[40]. The nanoparticles reduce the breakdown threshold of various metals ranging
from 19 to 35%, while no changes occur for semiconductors and insulators. They have
also demonstrated that the size (10–20 nm) and concentration (0.02–0.004 mg/mL)
of the Ag nanoparticles do not influence the metals’ emitted signal strength. The
authors also reported that NELIBS enabled detection lowers the limit of detection
(LOD) by more than 1 order of magnitude in the case of Mn and Pb in copper-based