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V. K. Unnikrishnan

Bio: V. K. Unnikrishnan is an academic researcher from Manipal University. The author has contributed to research in topics: Laser-induced breakdown spectroscopy & Spectroscopy. The author has an hindex of 15, co-authored 49 publications receiving 663 citations.


Papers
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Journal ArticleDOI
05 Sep 2010-Pramana
TL;DR: In this paper, a 355 nm pulsed Nd:YAG laser with a pulse duration of 6 ns focussed onto a copper solid sample in air at atmospheric pressure is studied spectroscopically, and the temperature and electron density characterizing the plasma are measured by time-resolved spectroscopy of neutral atom and ion line emissions in the time window of 300-2000 ns.
Abstract: Plasma produced by a 355 nm pulsed Nd:YAG laser with a pulse duration of 6 ns focussed onto a copper solid sample in air at atmospheric pressure is studied spectroscopically. The temperature and electron density characterizing the plasma are measured by time-resolved spectroscopy of neutral atom and ion line emissions in the time window of 300-2000 ns. An echelle spectrograph coupled with a gated intensified charge coupled detector is used to record the plasma emissions. The temperature is obtained using the Boltzmann plot method and the electron density is determined using the Saha-Boltzmann equation method. Both parameters are studied as a function of delay time with respect to the onset of the laser pulse. The results are discussed. The time window where the plasma is optically thin and is also in local thermodynamic equilibrium (LTE), necessary for the laser-induced breakdown spectroscopy (LIBS) analysis of samples, is deduced from the temporal evolution of the intensity ratio of two Cu I lines. It is found to be 700-1000 ns.

134 citations

Journal ArticleDOI
TL;DR: In this article, a Laser Induced Breakdown Spectroscopy (LIBS) technique has been applied for the identification of four widely used plastics, polyethylene terephthalate (PET), high-density polyethylenes (PE), polypropylene (PP) and polystyrene (PS), whose recycling is required from commercial and biosafety points of view.
Abstract: A Laser Induced Breakdown Spectroscopy (LIBS) technique has been applied for the identification of four widely used plastics, polyethylene terephthalate (PET), high-density polyethylene (PE), polypropylene (PP) and polystyrene (PS), whose recycling is required from commercial and biosafety points of view. The 3rd harmonic (355 nm) nanosecond pulse from an Nd:YAG laser is used to create plasma on the sample surface and identification of the type of the plastic is derived from the plasma emission. Principal Component Analysis (PCA) of the LIBS spectra is employed for the classification of plastics. Distinct methods have been used, apart from principal components of PCA, to further confirm our results. Statistical parameters, viz., Mahalanobis distance (M-distance) and spectral residuals were used for decisive match/no match test which provided successful classification of plastics. Receiver Operating Characteristic (ROC) and Youden's index analyses were carried out to obtain the diagnostic threshold for classification of all four classes of plastics. Sensitivity, specificity, predictive values and discriminative accuracy of the classification tests based on the optimum threshold were calculated. This proves the analytical predictive capabilities of the LIBS technique for plastic identification and classification. The technique of LIBS, in future, can be routinely used in field applications such as plastic waste sorting and recycling.

115 citations

Journal ArticleDOI
TL;DR: In this article, a method to fabricate flexible SERS substrates by replicating laser-written patterns created on polymethylmethacrylate (PMMA) onto a softelastomer, namely polydimethylsiloxane (PDMS), followed by in situ reduction of silver nanoparticles on the surface.
Abstract: The ability to fingerprint a few molecules via surface enhanced Raman scattering (SERS) continues to be of considerable utility in diverse fields encompassing physics, chemistry, materials sciences, nanotechnology, biomedicine, and environmental engineering. However, the development of facile and low cost approaches towards the fabrication of flexible substrates with very high SERS signal enhancement remains a challenge. Compared to conventional plasmonic-based sensors, a superhydrophobic plasmonic surface provides the combined advantage of concentration enrichment of solute molecules, the fourth power dependent localized electric field enhancement as well as the enhanced light scattering on rough surface. We demonstrate here a method to fabricate flexible SERS substrates by replicating laser-written patterns created on polymethylmethacrylate (PMMA) onto a soft-elastomer, namely polydimethylsiloxane (PDMS), followed by in situ reduction of silver nanoparticles on the surface. Laser writing of structures is fluence dependent and leads to substantial enhancement of Raman signals compared to a conventional plasmonic surface. Our fabricated surfaces provide a limit of detection of eight femtomolar for Rhodamine 6G upon 532 nm resonance excitation; an enhancement factor of ∼1010 is achieved for methyl orange. Studies of how water droplets spread on our laser-written surfaces indicate a fluence dependent enhancement in apparent contact angle with a concomitant increase in the contact angle hysteresis (CAH). Most significantly, our replicated patterns exhibit lotus effect (superhydrophobic surface with ultra-low contact angle hysteresis), that upon reduction of silver nanoparticles, exhibit superhydrophobicity with high contact angle hysteresis (rose petal effect). Our findings offer fresh opportunities for expanding the applicability of the SERS technique via superhydrophobic flexible plasmonic nanostructures.

57 citations

Journal ArticleDOI
TL;DR: The efficacy of a combined laser-induced breakdown spectroscopy (LIBS)–Raman system for the rapid identification and classification of post-consumer plastics is demonstrated and the potential limitations of any of these techniques for sample identification can be overcome by the complementarity of these two techniques.
Abstract: Classification of plastics is of great importance in the recycling industry as the littering of plastic wastes increases day by day as a result of its extensive use. In this paper, we demonstrate the efficacy of a combined laser-induced breakdown spectroscopy (LIBS)–Raman system for the rapid identification and classification of post-consumer plastics. The atomic information and molecular information of polyethylene terephthalate, polyethylene, polypropylene, and polystyrene were studied using plasma emission spectra and scattered signal obtained in the LIBS and Raman technique, respectively. The collected spectral features of the samples were analyzed using statistical tools (principal component analysis, Mahalanobis distance) to categorize the plastics. The analyses of the data clearly show that elemental information and molecular information obtained from these techniques are efficient for classification of plastics. In addition, the molecular information collected via Raman spectroscopy exhibits clearly distinct features for the transparent plastics (100% discrimination), whereas the LIBS technique shows better spectral feature differences for the colored samples. The study shows that the information obtained from these complementary techniques allows the complete classification of the plastic samples, irrespective of the color or additives. This work further throws some light on the fact that the potential limitations of any of these techniques for sample identification can be overcome by the complementarity of these two techniques.

56 citations

Journal ArticleDOI
12 Jun 2012-Pramana
TL;DR: The application of calibration-free laser-induced breakdown spectroscopy for quantitative analysis of materials, illustrated by CF-LIBS applied to a brass sample of known composition, is presented in this article.
Abstract: The application of calibration-free laser-induced breakdown spectroscopy (CF-LIBS) for quantitative analysis of materials, illustrated by CF-LIBS applied to a brass sample of known composition, is presented in this paper The LIBS plasma is produced by a 355 nm pulsed Nd:YAG laser with a pulse duration of 6 ns focussed onto a brass sample in air at atmospheric pressure The time-resolved atomic and ionic emission lines of Cu and Zn from the LIBS spectra recorded by an Echelle spectrograph coupled with a gated intensified charge coupled detector are used for the plasma characterization and the quantitative analysis of the sample The time delay where the plasma is optically thin and is also in local thermodynamic equilibrium (LTE), necessary for the elemental analysis of samples from the LIBS spectra, is deduced An algorithm relating the experimentally measured spectral intensity values with the basic physics of the plasma is developed Using the algorithm, the Zn and Cu concentrations in the brass sample are determined The analytical results obtained from the CF-LIBS technique agree well with the certified values of the elements in the sample, with an accuracy error <1%

53 citations


Cited by
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Journal ArticleDOI
TL;DR: Basic diagnostics aspects of laser-induced breakdown spectroscopy are focused on and a review of the past and recent LIBS literature pertinent to this topic is presented and previous research on non-laser-based plasma literature, and the resulting knowledge, is emphasized.
Abstract: Laser-induced breakdown spectroscopy (LIBS) has become a very popular analytical method in the last decade in view of some of its unique features such as applicability to any type of sample, practically no sample preparation, remote sensing capability, and speed of analysis The technique has a remarkably wide applicability in many fields, and the number of applications is still growing From an analytical point of view, the quantitative aspects of LIBS may be considered its Achilles' heel, first due to the complex nature of the laser–sample interaction processes, which depend upon both the laser characteristics and the sample material properties, and second due to the plasma–particle interaction processes, which are space and time dependent Together, these may cause undesirable matrix effects Ways of alleviating these problems rely upon the description of the plasma excitation-ionization processes through the use of classical equilibrium relations and therefore on the assumption that the laser-induced

835 citations

01 Jan 1999

643 citations

Book ChapterDOI
01 Jan 2018
TL;DR: Laser induced breakdown spectroscopy (LIBS) as discussed by the authors is a technique where atoms and ions are primarily formed in their excited states as a result of interaction between a tightly focused laser beam and the material sample.
Abstract: Laser induced breakdown spectroscopy (LIBS) is basically an emission spectroscopy technique where atoms and ions are primarily formed in their excited states as a result of interaction between a tightly focused laser beam and the material sample. The interaction between matter and high-density photons generates a plasma plume, which evolves with time and may eventually acquire thermodynamic equilibrium. One of the important features of this technique is that it does not require any sample preparation, unlike conventional spectroscopic analytical techniques. Samples in the form of solids, liquids, gels, gases, plasmas and biological materials (like teeth, leaf or blood) can be studied with almost equal ease.LIBS has rapidly developed into a major analytical technology with the capability of detecting all chemical elements in a sample, of real- time response, and of close-contact or stand-off analysis of targets. The present book has been written by active specialists in this field, it includes the basic principles, the latest developments in instrumentation and the applications of LIBS. It will be useful to analytical chemists and spectroscopists as an important source of information and also to graduate students and researchers engaged in the fields of combustion, environmental science, and planetary and space exploration. It features: recent research work, possible future applications and LIBS Principles.

611 citations

Journal ArticleDOI
TL;DR: This review is focused on recent developments of surface-enhanced Raman scattering (SERS) applications in Analytical Chemistry and covers advances in the fabrication methods of SERS substrates, including nanoparticles immobilization techniques and advanced nanopatterning with metallic features.

276 citations

Journal ArticleDOI
TL;DR: In this article, a review on the analytical results obtained by laser-induced breakdown spectroscopy (LIBS) is presented, including the risk of misclassification, and results on concentration measurement based on calibration are accompanied with significant figures of merit including the concept of accuracy.

236 citations