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Laser plasma spectrochemistry Richard E. Russo,*a Timothy W. Suen,a Alexander A. Bol’shakov,b Jong Yoo,b Osman Sorkhabi,a Xianglei Mao,a Jhanis Gonzalez,ab Dayana Oropezaa and Vassilia Zorbaa
Downloaded by Lawrence Berkeley National Laboratory on 21 June 2011 Published on 16 June 2011 on http://pubs.rsc.org | doi:10.1039/C1JA10107B
Received 22nd March 2011, Accepted 24th May 2011 DOI: 10.1039/c1ja10107b An overview of laser plasma spectrochemistry is presented to demonstrate its wide range of capabilities. Laser plasmas offer the ability to perform elemental, isotopic, molecular, quantitative and qualitative sample analysis with sub-micron spatial resolution, and each feature can be measured at standoff distances. Obviously, these attributes are not all achievable at the same time, but they can be optimized for specific applications. This manuscript gives a sampling (pun intended) of the research in our group that has demonstrated each of these capabilities. Although the technology is commonly referred to as LIBS (laser-induced breakdown spectroscopy), the authors prefer to use laser plasma spectrometry to represent the underlying science.
Introduction There is little doubt that laser ablation is gaining acceptance for direct solid sampling in chemical analysis.1–4 The benefits are well documented and include little or no sample preparation, no consumable or waste products, real-time measurements, excellent spatial resolution, and laboratory and field measurements. The two primary implementations of laser ablation include measuring optical emission from the laser induced plasma at the sample surface (a technology referred to as laser induced breakdown spectroscopy or LIBS) and by entraining the ablated a Lawrence Berkeley National Laboratory, University of California, Berkeley, CA, 94720, USA. E-mail:
[email protected] b Applied Spectra, Inc., 46661 Fremont Boulevard, Fremont, CA, 94538, USA
Dr. Russo is founder and scientific director of the laser material interactions group at the Lawrence Berkeley National Laboratory. His group has pioneered the development of laser ablation for chemical analysis, with an almost 30 year contribution to fundamental and applied research topics, with over 200 Richard E: Russo refereed scientific publications. Fourteen students have received their PhD degree under his direction at the University of California, Berkeley. Dr. Russo also is president and founder of Applied Spectra, Inc. The company manufactures analytical instruments using LIBS and Laser Ablation with ICP-OES and ICP-MS. This journal is ª The Royal Society of Chemistry 2011
mass in a gas flow with transport to an inductively coupled plasma with mass (ICP-MS) or emission (ICP-OES) detection. The heart of the technology is ablation (removing or sampling) of a small portion of the sample using a pulsed laser beam. Research into the mechanisms of the ablation process has provided advances in its analytical performance.5,6 Plasma formation and expansion have been investigated to demonstrate spectral line to continuum behavior versus temperature and electron number density over time.7–9 Particle ejection and formation also have been addressed for improving analytical performance when using ICP detection.10–12 Without going into detail on specific mechanisms, UV wavelengths and short laser pulses generally provide the best performance metrics in terms of precision, accuracy, and sensitivity when ablation is coupled with ICP for analysis, as has been experienced by the geological community.13,14 However, good analytical chemistry easily can be achieved using IR nanosecond pulses; most LIBS research and analysis is performed using the 1064 nm Nd:YAG laser with a nanosecond pulse duration.15,16 The ‘optimal’ laser conditions really depend on the sample, the availability of calibration or reference materials, and most importantly, the purpose of the analysis. This manuscript will describe characteristics of the laser induced optical plasma as a modern day source for spectrochemistry. Only the capabilities of optical emission at the sample surface will be addressed; not the detection of the ablated particles using the ICP with OES or MS. Many manuscripts exist to describe laser ablation for ICP, including several as referenced below,1–4 including a new review article.1 Impressively, this laser ablation plasma source has been demonstrated for elemental, isotopic, molecular characterization, and qualitative and quantitative analysis. In addition, capabilities include sub-micron spatial resolution and open-path stand off measurements. The authors are not aware of any other analytical technology that provides such diversity of capabilities. This manuscript will J. Anal. At. Spectrom.
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describe studies representing several of the capabilities of the laser plasma source for analytical chemical analysis. Specific applications of qualitative and quantitative analysis are not covered and can be found in several recent references 17–20.
Downloaded by Lawrence Berkeley National Laboratory on 21 June 2011 Published on 16 June 2011 on http://pubs.rsc.org | doi:10.1039/C1JA10107B
Nanoscale laser ablation in the optical near-field21,22 The roadmap for innovation from the NSF, DOE and NIH encourages the development of new tools (capabilities) for performing chemical imaging and analysis with nanoscale spatial resolution. Analytical technologies such as SIMS (Secondary Ion Mass Spectroscopy) and Auger Spectroscopy that provide such spatial resolution involve vacuum and electron beams. Laser ablation chemical analysis has the potential to become a revolutionary tool for nanoscale elemental analysis without requiring a vacuum chamber. Preliminary research on brain tissue mapping shows the potential for this technology.23 In general, the use of laser beams for ablation in chemical analysis has a fundamental restriction in the spatial resolution that can be achieved, imposed by the diffraction limit; light cannot be focused to dimensions lower than roughly half the wavelength of the irradiation used, which for conventional laser systems is on the order of a few hundred nanometres. Overcoming this limitation is possible by utilizing non-conventional optical lithography structuring schemes which take advantage of near-field effects.24–26 When a sample is placed in the near field, the irradiation is physically confined in its immediate vicinity through exploitation of evanescent or non propagating fields. This approach results in sub-diffraction limited resolution, which in conjunction with femtosecond laser pulses constitutes one of the basic strategies for materials processing on the nanoscale level. We configured a Near-field Scanning Optical Microscope (NSOM) for ablation experiments (Fig. 1a and b). Laser light incident at the input of a fiber is guided to its tapered output end27–30 which has an opening (aperture) that typically ranges from a few tens to a few hundred nanometres (Fig. 1c). Control over the tip–sample distance is achieved via tuning fork normal force feedback and is kept at 8–10 nm.31 Fig. 2a shows an AFM surface map of nano-features created using 400 nm, 100 fs laser pulses with an output energy of 0.23 nJ from the NSOM probe. Each row represents regions ablated with a fixed number of laser pulses in the range of 5 to 500, so as to assess reproducibility. A regime which separates the formation of different types of
Fig. 2 (a) AFM surface map of a near-field laser processed Si surface using the 400 nm, 100 fs laser source. (b) Line profile for 500 pulses and (c) for 10 pulses. (d) Dependence of lateral feature size, full width at half maximum (FWHM) and height or depth of the structures on the number of laser pulses for an input energy of 0.18 nJ. The dashed lines are guides. (e) The smallest crater ablated using a single fs pulse at 400 nm and 0.18 nJ. The FWHM size is 27 nm, the diameter is 55 nm with a depth of 1.2 nm.
surface features is identified (red line in Fig. 2a) after approximately 20 pulses; for low numbers of pulses, craters are formed (line profile Fig. 2c) in the bulk, while for higher numbers of pulses these structures are transformed into protrusions above the surface (line profile Fig. 2b). The dependence of the lateral diameter, the full width at half maximum (FWHM) and the average height (for protrusions) or depth (for craters), are shown in Fig. 2d. Under these conditions, 1.2 nm deep craters with approximately 27nm FWHM were ablated (Fig. 2e). These features generated using near-field laser ablation are among the smallest ever reported. However, we are not able to measure optical emission at this spatial scale at this time. Two possible reasons are that optical emission does not exist under these conditions, or if it does, we need better detection capabilities to measure light within the near-field. Currently, we use far-field optics for detection. Research is underway to design near-field based secondary excitation and detection capabilities in order to perform analytical chemistry at this scale.
Submicron scale fs laser ablation-based chemical analysis21,32
Fig. 1 Near-field Scanning Optical Microscope (a, b) that has been modified to accommodate the use of fs laser pulses. (c) Light emitted from the end of a near-field NSOM probe (end diameter
