In order to realize surface elemental microanalysis of solid samples with submicron lateral resolution, laser-ablation (LA) combined with high sensitive laser-induced fluorescence (LIF) detection was investigated. A 532 nm or 266 nm nanosecond laser pulse with low pulse energy was used to realize submicron laser-ablation on the surface of a copper alloy, and LIF technique was used to sensitively detect a minor lead element in the ablated samples. ~344 nm and ~267 nm lateral resolutions could be achieved experimentally under 532 nm and 266 nm laser ablations under the current experimental condition, respectively. This demonstrated the feasibility of using a LA-LIF technique for surface elemental microanalysis of solid samples with submicron spatial resolution. The potentials of continually improving the spatial resolution of this technique to nanoscale were discussed.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The analysis of elemental distribution under high spatial resolution for solid samples is very important for help understanding their physical, chemical, or biological properties and it is widely required in sample analysis of many fields, such as material science , metallurgy , biological science [3, 4] and archaeology [5, 6] etc. For two-dimension (2D) elemental analysis or 2D elemental mapping technology, lateral resolution is a key parameter. Higher lateral resolution is helpful to get more localized information about the elemental distribution on the sample surface.
Some conventional techniques can be used for elemental analysis of sample surface under high lateral resolution , such as Auger electron spectroscopy (AES) [8, 9], X-ray photoelectron spectroscopy (XPS)  and secondary ion mass spectroscopy (SIMS) . However, these techniques require a high vacuum condition and the sample sizes are usually limited. Although energy-dispersive X-ray spectroscopy (EDS) only requires moderate vacuum condition, analysis of nonconductive samples is not direct, where a thin coating of carbon or gold on the sample surface is usually required .
Two laser-ablation-based surface elemental analysis techniques have been developed in the past decades. The first one is laser-ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS) and the second one is laser-induced breakdown spectroscopy (LIBS). In comparison with electron beam or X-ray based elemental analysis techniques, LA-ICP-MS and LIBS have the following advantages, such as possibilities of operation at ambient atmosphere, direct analysis of non-conducting samples, and the capability of analyzing light elements etc . For this reason, LA-ICP-MS and LIBS have been developed for mapping of trace elements in plant tissues , garnets  and nuclear waste glass . Limbeck et al. reported the combination use of LA-ICP-MS and LIBS to realize laterally resolved elemental analysis of biological samples .
In conventional single-pulse LIBS (SP-LIBS) studies, the lateral resolutions are usually at the level of several to tens of microns depending on applied laser pulse energy and the focusing condition in the experiments [16–18]. In order to improve the lateral resolution, femto-second laser can be used as laser source of LIBS due to its short pulse duration and less thermal effect in the laser-ablation process . A lateral resolution of ~1 μm was reported by Banerjee et al. when 266 nm femto-second laser was focused on the chromium layer coated on silicon substrate by microscope objective with the numerical apertures of 0.28 . A lateral resolution of 450 nm was also reported by Zorba et al. when 400 nm femto-second laser was focused by a microscope objective with numerical apertures of 0.45 . Kossakovski and Beauchamp applied near-field LIBS technique to realize surface elemental microanalysis under high spatial resolution . However, the atomic emission signal became hard to detect if higher spatial resolution was required. The reason is that the dependence of spatial resolution on laser pulse energy and the dependence of analytical sensitivity on laser pulse energy in SP-LIBS are opposite. Due to both sample ablation and plasma formation processes are controlled by the same laser pulse, it is impossible to optimize both of them by setting adequate laser pulse energy each time.
Orthogonal double-pulse LIBS (DP-LIBS) is able to solve this problem existed in SP-LIBS . In orthogonal DP-LIBS, the first laser pulse with low pulse energy is selected to form small crater on the sample surface, and the second time-delayed laser pulse with moderate pulse energy is used to enhance atomic emission intensity for sensitive spectral analysis. Although the atomic emission from the ablated samples in the plasma can be improved in orthogonal DP-LIBS technique, due to the low excitation efficiency of atoms on the ground state and the influence of continuum background to the signal detection, the detection sensitive to the ablated samples are still limited, thus the obtained lateral resolution of the orthogonal DP-LIBS is still at micron scale .
Spark discharge assisted LIBS (SD-LIBS) or laser-ablation spark-induced breakdown spectroscopy (LA-SIBS) techniques face the same problem as in orthogonal DP-LIBS on signal detection [24, 25]. Laser-induced fluorescence (LIF) detection is possible the most efficient optical method can be used to improve detection sensitivity. Due to the capability of resonant and high efficient excitation, transition selectable and almost zero-background line detection, LIF is able to realize very high sensitive detection for the formed atoms in the laser-induced plasma. For this reason, LA-LIF (most time also called LIBS-LIF) technique has been applied to analyze the trace elements in solids [26–28], liquids [29, 30] and aerosol samples  with high analytical sensitivity. Cheung’s group developed laser excited atomic fluorescence (LEAF) technique to realize high sensitive elemental analysis for different samples [32–34]. In this technique, ArF laser at 193 nm laser but not tunable laser was used to re-excite the atoms in laser-induced plasma. Although LA-LIF or LEAF has been widely applied on high sensitive elemental analysis, no literature reports can be found about the application of this technique on the surface elemental microanalysis under submicron lateral resolution.
In this work, LA-LIF technique was demonstrated to realize minor lead element (clead = 1.5%) microanalysis for copper alloy under submicron lateral resolution for the first time. A Q-switched Nd:YAG laser with low energy was used to ablate small amount of samples, then a time-delayed tunable dye laser was used to resonantly excite the atoms on the ground state and atomic emission from the excited atoms was selectively detected for high sensitive detection. 532 nm and 266 nm lasers were used as the ablation laser and the obtained lateral resolutions were compared. The strategy of further improving the lateral resolution of this technique to realize nanoscale elemental microanalysis was also discussed.
The LA-LIF experimental setup of this work is shown in Fig. 1. The ablation laser at wavelengths of 532 nm or 266 nm was generated from an electro-optically Q-switched Nd:YAG laser operated at 5 Hz repetition rate. The M2 parameter of the fundamental output of this laser at 1064 nm was 2.4. The pulse width (FWHM) of the laser was ~12 ns. The ablation laser beam passed through two Glan-Taylor linear polarizers (P1 & P2) and was then tightly focused on the sample surface by a 50 × microscope objective (N.A = 0.42) or 10 × UV microscope objective (N.A = 0.25) from the direction perpendicular to the sample surface. The first Glan-Taylor polarizer P1 was used to enhance the polarization degree of the ablation laser beam and the second Glan-Taylor polarizer P2 was used to adjust the laser pulse energy of the ablation laser beam by rotating its polarization angle relative to that of P1, thus the pulse energy of the ablation laser could be controlled precisely. The pulse energy of the ablation laser beam can be calculated according to
The LIF laser was generated by a tunable home-made dye laser system followed with a second harmonic generation (SHG) which was described in detail elsewhere . The LIF laser was focused on to the tiny plasma produced by the ablation laser with a plano-convex lens L1 (quartz, f = 150 mm). The wavelength was tuned to the resonant wavelength of lead at 283.31 nm. The propagation direction of the LIF laser beam was parallel to the sample surface. The focal spot was located just in front of the sample surface and the distance from the focal spot to sample surface must be carefully adjusted to avoid sample ablation by the LIF laser. This could be identified by observing if the atomic emission from sample atoms appeared when the ablation laser was blocked. The time delay between the LIF laser pulse and the ablation laser pulse was controlled by a four-channel pulse delay generator (Quantum Composers, 9520).
The sample was copper alloy which was cut into the shape of a hexagonal prism, as shown in the top right corner of Fig. 1, and the width of the sample surface along the propagation direction of LIF laser beam was only 3 mm. This could avoid the blocking of the LIF laser beam by the edge of the sample. The sample was mounted on an x–y motion platform and was kept a moving speed of 150 μm/s during experiments, thus ensuring that each crater was formed by single laser shot.
The emission of the plasma and lead atoms was collected and collimated by quartz lens L2 (f = 150 mm), then reflected by folding mirror RM, and finally focused on to the entrance slit of a 50 cm monochromator (Tianjin Tuopu Instruments, WDS-5) by quartz lens L3 ((f = 250 mm)). The widths of both entrance and exit slits were set at 100 µm, thus the spectral resolution of this monochromator was better than 0.1 nm. A photomultiplier tube (PMT) (Hamamatsu, CR114) was used as photo detector. The output signal of the PMT was recorded with a 500 MHz digital storage oscilloscope (Gw Instek, GDS-3502), which was also triggered by the 9520 pulse delay generator.
In order to measure the radius of craters formed by ablation laser under different pulse energies, the surface of the copper alloy sample was polished carefully with No. 5000 sand paper before experiment to make sure good flatness. During the experiment, the pulse energies of LIBS laser were accurately controlled by rotating the second polarizer P2 to form tiny craters on sample surface, and then the sample surface was observed and photographed with a scanning electron microscope (Hitachi, 1530N) and the profiles of the craters were analyzed with a three-dimension (3D) profile analyzer (RTEC Instruments).
3. Results and discussion
3.1 Signal enhancement
Figure 2(a) shows the energy levels of lead relative to LIF detection. The lead atoms were excited resonantly from the ground state to the upper state by frequency-doubled dye laser at 283.31 nm. Then the excited lead atoms spontaneously decay from state to state, the fluorescence at 405.78 nm from this transition was monitored in spectral analysis.
Figure 2(b) shows typical temporal profiles of lead atomic emission at 405.78 nm and the plasma background emission at 404.0 nm recorded under laser ablation with and without existing LIF excitation. The pulse energy of 532 nm ablation laser and LIF laser was 40 µJ and 110 µJ, respectively. The interpulse time delay between them was 1.5 µs. Both atomic emission of lead and background emissions of the plasma could be observed in time period of 0-1.0 µs in both cases. However, due to the low pulse energy of the ablation laser, the atomic emission of lead was quite weak and almost immerged into the background; therefore, the signal to background ratio was quite low in the case of laser ablation only. However, a strong fluorescence with ~30 ns time duration appeared at ~1.5 µs after LIF laser was used to excite the lead atoms in the plasma. In LA-LIF case, not only the signal intensity was enhanced significantly, but also the background decreased to almost zero leading to background-free line detection. The signal-to-background ratio has been improved significantly in comparison with that obtained in the case of laser ablation only. This is very helpful to realize ultrasensitive detection of the ablated samples. It is worth to mention that the 1.5 µs time delay was the optimized time delay under current experimental condition after comparing the signal to background ratio under different interpulse time delays experimentally.
3.2 Experimental observations
The radius of the crater (rt) generated by focused Gaussian laser beam and the laser pulse energy (E) satisfy the following relation ,
This indicated that the radius of crater will decrease with the decreasing of the pulse energy of the ablation laser.
3.2.1 Ablation with 532 nm laser
The 532 nm laser was first selected as the ablation laser in the experimental studies. A 50 × long working-distance (W.D. = 20.5 mm) microscope objective with 0.42 numerical apertures was used to focus the ablation laser beam. Figure 3(a) shows a plot of the LIF peak signal intensities of lead versus the pulse energy of 532 nm ablation laser in LA-LIF technique, where, the pulse energy of the LIF laser was fixed at ~110 μJ and the interpulse time delay was 1.5 μs. Under low pulse energy for the ablation laser, the LIF signals intensity increased linearly with the laser pulse energy; however, the LIF signal showed a “saturation” effect under the laser-ablation with slightly higher laser pulse energy. This is possibly due to the enlarged plasma volume and only the atoms in the focus of the LIF laser beam can be excited. For this reason, only the experimental data points obtained under laser ablation with pulse energy below 12 μJ are selected to give a linear fitting. The lowest pulse energy of the ablation laser applied in the measurements was 0.5 μJ. Under this lowest pulse energy, the LIF peak signal intensity was 5 times of the standard deviation of the background σB. Here, σB was evaluated based on 10 repeated background measurements monitored at 404.0 nm.
It is necessary to measure the radius of the craters formed by ablation laser with different pulse energies for evaluating the spatial resolution capability of LA-LIF technique. Figure 3(b) shows a plot of the radius of the craters versus the pulse energy of 532 nm ablation laser. The experimental data points can be well fitted with Eq. (3) under low and medium pulse energies. However, the deviation of the experimental result from the fitted curve goes bigger under high laser pulse energies due to that the real laser beam is not ideal Gaussian beam. Figure 4(a) shows the scanning electron microscope (SEM) photo of craters formed on copper alloy surface, the four column craters are formed by the laser with pulse energy corresponded with the four lowest one showing in Fig. 3(b). The smallest crater, formed by the ablation laser with 0.5 μJ pulse energy was shown in Fig. 4(b) and its diameter was measured to be ~800 nm.
It is well adopted that the crater size (FWHM) can be considered as the lateral resolution in the surface elemental analysis with LIBS [20, 21]. In order to have the crater size, only measuring the maximum diameter with SEM is not enough; the depth profile of the crater has to be analyzed with 3D prolife analyzer. Figure 5 shows the depth profile of the craters formed by 532 nm laser with different pulse energies in XOY plane. The measured maximum diameters, the crater sizes and depths of the craters are listed in Table 1. According to these measurements, the averaged ratio of the crater size to the maximum diameter of the crater is ~0.43. The craters formed by the ablation laser with pulse energies lower than 1.3 µJ have not been analyzed with the 3D prolife analyzer. Alternatively, the size of the crater shown in Fig. 4(b) was roughly estimated by taking account of the same ratio of the crater size to the maximum diameter, 0.43. Thus the estimated value was ~344 nm, which represented the experimentally observed lateral resolution in this study under 532 nm laser-ablation.
3.2.2 Ablation with 266 nm laser
The 266 nm laser was then selected as the ablation laser in the experimental studies. A 10 × UV microscope objective with 0.25 numerical apertures and 15 mm working distance (Thorlabs Inc., LMU-10X-266) was used to focus the ablation laser beam. Other experimental conditions were all the same with that in LA-LIF with 532 nm laser. Figure 6(a) shows a plot of the LIF peak signal intensities of lead versus the pulse energy of 266 nm ablation laser, and the linear fitting curve under the low ablation laser energy was also given. The lowest pulse energy of the ablation laser used was 0.6 μJ. Under the laser-ablation with 266 nm laser with this low energy, the peak LIF signal intensity of lead was 6 times as strong as σB.
Similarly, the craters formed on copper alloy surface by 266 nm laser with different pulse energies was imaged by SEM and the diameters of them were measured. Figure 6(b) shows a plot of the radius of the craters versus the pulse energy of 266 nm ablation laser. The data points can also be fitted according to Eq. (3) and linear fitting can be made for several data points under low pulse energies. Figure 7 shows the SEM photo of the crater formed by the ablation laser with 0.6 μJ pulse energy. The diameter of this crater is measured to be ~620 nm.
If taking account of the same ratio of the crater size to the maximum diameter obtained in the laser-ablation with 532 nm laser, the crater size will be estimated to be ~267 nm. It should be explained that although this ratio was not experimentally determined in the case of laser ablation with 266 nm laser, ~0.43 should be a reasonable value. If just simply assuming the carter is circular cone shaped, this value will be 0.5, and the relative error would be 14%. One of the precise methods to determine the crater size is atomic force microscope (AFM). The purpose of this article was to demonstrate the feasibility of using LA-LIF technique as a microanalysis tool under high spatial resolution, less attentions was paid on the measurement accuracy on the spatial resolution. If necessary, the crater size will be precisely measured with AFM in future studies.
Generally, a signal is thought to be detectable when its peak intensity is 3 times of the standard deviation of the background σB, according to 3σ rule. Under the laser-ablation with 532 nm or 266 nm laser at the minimum laser pulse energies, the signal is 5 times or 6 times of the corresponding background σB, respectively. Therefore, the final possible lateral resolution of this technique should be better than the experimentally observed lateral resolution reported here. Higher lateral resolution has been realized by using 266 nm ablation laser than 532 nm ablation laser even though the numerical apertures of the two microscope objective were different. The final achievable lateral resolution of this technique is also depend on the concentration of the analyzed elements. The lateral resolution for analyzing major elements will be higher than that for analyzing minor or trace elements in the samples under the same experimental conditions.
The limit of detection of an element (concentration) is relevant with the lateral resolution set by the experimental condition because the ablated sample mass is determined by the crater volume, and the crater volume is relevant with the lateral resolution. Therefore, it better to report one by fixing another between limit of detection and lateral resolution. For example, in this work, we can say for analyzing lead in copper alloy at the concentration of 1.5%, the achievable lateral resolution is ~344 nm under the laser ablation with 532 nm laser. Inversely, we can say when the lateral resolution is set at 344 nm; the limit of detection of lead in copper alloy will be better 1.5%.
To further improve the lateral resolution of this technique, two aspects should be considered: The first one is continually decreasing the diameters of the craters formed by laser-ablation on sample surface. One strategy is selecting shorter wavelength for the ablation laser and UV microscope objective with higher numerical apertures . The second strategy is modifying the wave fronts or polarization of the ablation laser beam to give better focusing and form smaller craters. For example, by using optical eigenmode approach and radial polarized Bessel beam, much smaller spot size can be realized under tight focusing condition [38–40].
The second aspect should be considered is continually improving the detection sensitivity for the ablated samples. Increasing the pulse energy and reducing the line width of the LIF laser will be helpful to increase excitation efficiency of the atoms from the ground state to the upper state hence to enhance the signal intensity of the LIF. It’s worthwhile to mention that under the laser-ablation with very low laser pulse energies, the sample breakdown or atomization process might not completely happen. In this case, it is possible to introduce another laser pulse with moderate pulse energy from orthogonal direction relative to both direction of ablation laser and LIF laser beams to deeply breakdown the ablated samples. The technique can be termed as LA-LIBS-LIF and it is possible to make positive contribution to signal enhancement of the LIF under the laser ablation with very low pulse energies. After these, it is believed that the LA-LIF technique will be possible to achieve nanoscale spatial resolution on the surface elemental analysis of solid samples.
State-of-the-art LA-ICP-MS technique is widely used on surface elements mapping of solid samples. In this technique, the ablated samples were delivered into the ICP-MS system by inert gas and then analyzed quantitatively. This analysis can’t be in situ and the lateral resolution is limited in micron scale while analyzing minor elements. In comparison with LA-ICP-MS technique, LA-LIF technique is possible to give in situ and high sensitive elements analysis with nanometer lateral resolution and the system is less cost than that of LA-ICP-MS. It could be a good and complementary technique for surface elements mapping or microanalysis of solid samples under high spatial resolution in the future.
In this article, the feasibility of using LA-LIF technique to realize surface elemental microanalysis of minor lead in copper alloy with submicron spatial resolution was first demonstrated. In LA-LIF, submicron scale craters could be formed by laser-ablation with very low pulse energies, and high sensitive detection of the ablated samples could be realized with LIF technique. In the experiment, copper alloy was ablated by either 532 nm or 266 nm pulsed laser, the minor lead element (c = 1.5%) in copper alloy could be analyzed under high lateral resolution. For minor lead microanalysis in copper alloy, ~344 nm and ~267 nm lateral resolutions have been achieved experimentally under current experimental condition corresponding to the laser ablation with nanosecond 532 nm and 266 nm lasers, respectively. More efforts should be made to improve the analytical sensitivity and continually decrease the crater size to improve the lateral resolution of this technique to nanoscale. LA-LIF technique could be a complementary technique of LA-ICP-MS on the surface elemental microanalysis or mapping under high spatial resolution. It will find wide applications on elemental microanalysis of solid samples in different areas, such as metallurgy, material, biological and environmental sciences etc.
National Basic Research Program (973 Program) of China (2012CB921900), and National Natural Science Foundation of China (11274123 & 11304100).
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