Qualification: 
Ph.D, MSc, M.Tech
m_sivakumar@cb.amrita.edu

Dr. Sivakumar M. currently serves as Associate Professor and Vice Chairperson in the Department of Sciences, School of Engineering, Coimbatore Campus. His areas of research include ultrafast laser processing of materials, Nanoscience and Nanotechnology.

Education

  • Ph.D. in Materials Engineering
    Instituto Superior Téchnico, Lisbon, Portugal.

Post Doctorial Study

  • Department of Aerospace Engineering, Ryerson University, Toronto, Canada
  • Department of Mechanical Engineering, UNSW, Sydney, Australia

Research Grants

Ongoing

Completed

Research Scholars

  1. Dr. Sreedevi Mohan (submitted)
  2. Ms. DivyaSree (Ongoing)

Publications

Publication Type: Journal Article

Year of Publication Publication Type Title

2019

Journal Article

S. Mohan, A, U. Maheswari, and Dr. Sivakumar M., “Structure dependent dielectric response of spray coated nanostructured zirconia thin films”, Materials Research Express, 2019.[Abstract]


In this work the structural dependent dielectric property of ZrO<sub>2</sub> nanoparticles and films prepared using these nanoparticles is studied. The tetragonal and monoclinic dominant zirconia nanoparticles were obtained through thermal treatment of PVP / CTAB capped hydrous amorphous zirconia (HAZ) at 500 <sup>o</sup>C. The grain (R<sub>g</sub>) and grain boundary (R<sub>gb</sub>) resistances of both types of nanoparticles along with films are estimated by fitting the respective complex impedance (Nyquist) plots with an equivalent electrical circuit. The nanostructured tetragonal dominant zirconia films are highly conductive as compared to monoclinic films due to large amount of Zr<sup>3+</sup> ions present at tetragonal sites. Consequently the film exhibits a high dielectric constant at lower frequency range due to Maxwell-Wagner polarization. Moreover the EPR signals observed at 1.946 (g<sub>||</sub>) and 1.967 (g<sub>⊥</sub>) reveals the presence of greater Zr<sup>3+</sup> ions with tetragonal films. The Rgb of monoclinic dominant film is increased by an order of 103 and consequently results in significant reduction of dielectric constant. Further the shift of loss tangent peak to higher frequency side of tetragonal dominant films endorses the higher mobility of charge carriers as that of monoclinic films. The enhancement of dielectric properties shows the potential application of tetragonal dominant ZrO<sub>2</sub> films for future electronic devices.

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2018

Journal Article

S. Mohan, A, U. Maheswari, and Dr. Sivakumar M., “Effect of Sintering Temperature on Dielectric Properties of Hydrous Amorphous Zirconia”, Materials Today: Proceedings, vol. 5, no. 8, pp. 16598-16609, 2018.[Abstract]


The effect of sintering temperature on structural and dielectric properties of Hydrous Amorphous Zirconia (HAZ) synthesized through surfactant assisted chemical precipitation method is investigated. X-ray diffraction studies reveal that sintering temperatures greater than lowest crystallization temperature (500 °C) of HAZ caused an increase in lattice spacing resulting in monoclinic phase dominant ZrO2 nanocrystals. The room temperature impedance measurement of pelletized hydrous amorphous zirconia powder sintered at 500 and 800 °C is carried out in the frequency range of 40 HZ to 5 MHZ. From the impedance data, the dielectric constant, dielectric loss, loss tangent and AC conductivity of HAZ sintered at these temperatures are calculated. It is found that dielectric constant is higher for sample sintered at lower crystallization temperature due to higher tetragonal content. This is attributed to higher concentration of oxygen vacancies which results in large space charge polarization and high dielectric constant. Further the loss tangent curve indicates that mobility of charge carriers is decreased in HAZ sintered at higher temperature and hence the conductivity also decreases. However at higher frequency region, the dielectric constant of monoclinic zirconia is greater than tetragonal zirconia.

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2018

Journal Article

M. V. P, Kumar, V. Senthil, A, U. Maheswari, Dr. Sivakumar M., S Kumar, S., and Rani, A., “Characterization and anticorrosion studies of spray coated nickel oxide (NiO) thin films”, Materials Today: Proceedings, vol. 5, no. 2, pp. 8577-8586, 2018.[Abstract]


In the present work, nanostructured thin films of NiO were synthesized using spray coating technique. The morphology and composition of the film were analyzed. The UV Visible absorption, micro Raman, and Photoluminescence spectrum of the NiO thin films were recorded and analyzed. The electrochemical behaviour of the nickel oxide thin film was examined by means of Electrochemical Impedance Spectroscopy (EIS) analysis, Tafel analysis and Open Circuit Potential (OCP) measurements. Salt spray corrosion test was applied to assess the stability of the NiO thin film over the steel substrate in the corrosive salt environment. The electrochemical impedance spectroscopy (EIS) analysis, Open Circuit Potential measurements (OCP), Tafel analysis revealed the enhanced protection of the substrate by nickel oxide thin film. The salt spray test carried out for 390 h, the thin film exposed to salt spray confirmed that the NiO films provides effective protection against corrosion of the stainless steel SS 304L compared with the substrate without nanoparticle deposition. Electrochemical Impedance Spectroscopy (EIS) analysis, Tafel analysis, Open Circuit Potential measurements (OCP) were repeated after the salt spray corrosion test and the result were again recorded, analyzed and compared with previous results which infers the effective protection of the nickel oxide thin film against corrosion in a corrosive environment..

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2018

Journal Article

M. V. P, S, S. Kumar, Kumar, V. Senthil, A, U. Maheswari, Mohan, S. R., Dr. Sivakumar M., and K Rani, A., “Enhancement of anticorrosion properties of stainless steel 304L using nanostructured ZnO thin films”, AIMS Materials Science, vol. 5, pp. 932-944, 2018.[Abstract]


Nanostructured ZnO thin films were coated on stainless steel specimen (304L SS) by depositing nanoparticles of zinc oxide. The morphology and optical properties of the thin films were epitomized using Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive Analysis of X ray (EDAX), Atomic Force Microscopy (AFM) and Photoluminescence spectroscopy (PL) techniques. FESEM and AFM images revealed that the present depositing procedure is extremely proficient to synthesize uniform and homogeneous nanostructured thin films. The presence of excitonic peak in the PL emission spectrum confirmed the nanocrystalline nature of the thin films. The anticorrosion nature of the zinc oxide coated stainless substrate in the brackish environment was studied using Tafel, Electrochemical Impedance Spectroscopy analysis and Open Circuit Potential studies (OCP) methods. The E corr , I corr , corrosion rate before and after salt spray were calculated from Tafel plot. These parameters indicated that the anticorrosion properties of coated thin films are substantially higher to that of bare steel. The Nyquist plot before and after salt spray was fitted using an equivalent circuit and the coating resistance R ct was calculated. The different mechanisms involved in the corrosion behavior of the thin films were discussed on the basis of equivalent circuit. The physical stability of the coated samples in saline surroundings was studied by AFM assisted nanoindentation techniques. The absence of the cracks and blisters in the sample after nanoindentation before and after salt spray revealed the adherent nature of the nanostructured thin films. 933 AIMS Materials Science Volume 5, Issue 5, 932-944.

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2017

Journal Article

U. Maheswari A, Mohan, S., and Dr. Sivakumar M., “Effect of Surfactant and Mineralizer on the Dielectric Properties of Zirconia Nanocrsytals”, Applied Surface Science, vol. 427, pp. 1174-1182, 2017.[Abstract]


The combined effect of surfactants (PVP/CTAB) and alkaline mineralizers (NaOH/NH4OH) on dielectric properties of zirconia nanocrystals is analyzed. It is found that, the stabilization of zirconia tetramers by surfactants and the rate of hydroxyl ions released by alkaline mineralizers have significant impact on the dielectric properties of nanocrystals. The PVP capped tetramers form highly conducting grains with insulating boundaries, whereas the grains of CTAB capped tetramers are highly insulating with conducting grain boundaries, as revealed by Nyquist plots. Consequently, the space charge polarization would be quite large in highly conducting grains resulting in higher dielectric constant values at lower frequencies. The higher dielectric constant of PVP capped nanocrystals is due to greater tetragonal coordination of 3d5/2 and 3d3/2 electrons of Zr4+ ions than that of CTAB capped nanocrystals. Further, the surface oxygen vacancies of PVP samples are higher, resulting in a high space charge polarization. The ESR signal corresponding to F+ centers appears stronger for PVP/NH4OH nanocrystals. Moreover, the larger ESR line width of PVP/NH4OH nanocrystals corresponding to more oxygen vacancies is in accordance with the inference attained from the XPS analysis.

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2017

Journal Article

M. V. P, S, S. Kumar, Kumar, V. Senthil, K, U. Maheswari, Dr. Sivakumar M., and Rani, A., “Investigation of anticorrosion properties of nanocomposites of spray coated zinc oxide and titanium dioxide thin films on stainless steel (304L SS) in saline environment”, Materials Research Express, vol. 5, 2017.[Abstract]


The present study reports the anticorrosive nature of nanocomposite thin films of zinc oxide and titanium dioxide on steel substrate (304L SS) using spray coating method. The morphology and chemical constituents of the nanocomposite thin film were characterized by field effect scanning electron microscopy and energy dispersive analysis of x-ray (EDAX) studies. From the EDAX studies, it was observed that nanocomposite coatings of desired stoichiometry can be synthesized using present coating technique. The cyclic voltametric techniques such as Tafel analysis and electrochemical impedance spectroscopy (EIS) analysis were conducted to study the anticorrosion properties of the coatings. The E corr values obtained from Tafel polarization curves of the sample coated with nanocomposites of ZnO and TiO2 in different ratios (5:1, 1:1 and 1:5) indicated that the corrosion resistance was improved compared to bare steel. The coating resistance values obtained from the Nyquist plot after fitting with equivalent circuit confirmed the improved anticorrosion performance of the coated samples. The sample coated with ZnO: TiO2 in the ratio 1:5 showed better corrosion resistance compared to other ratios. The Tafel and EIS studies were repeated after exposure to 5% NaCl for 390 h and the results indicated the anticorrosive nature of the coating in the aggressive environment. The root mean square deviation of surface roughness values calculated from the AFM images before and after salt spray indicated the stability of coating in the saline environment.

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2017

Journal Article

S. R. Mohan, Maheswari, A. U., and Dr. Sivakumar M., “Structural transformation of polymeric and cationic capped zirconia nanoparticles with temperature”, International Journal of Nanoparticles, vol. 9, pp. 88-100, 2017.[Abstract]


The phase transformation mechanisms of zirconia nano crystals obtained on thermal treatment of polyvinyl pyrrolidone (PVP) and cetyltrimethylammoniumbromide (CTAB) capped hydrous amorphous zirconia (HAZ) on thermal treatment are investigated. In PVP assisted synthesis, tetragonal zirconia nanoparticles formation is the predominant one as compared to CTAB assisted synthesis. The decomposition of surfactants capped on HAZ with temperature (526°C for PVP and 466°C for CTAB) and subsequent interaction of the by-products with zirconia nanoparticles are responsible for tetragonal to monoclinic phase transformation. The decomposition temperature of capped surfactants is observed from thermogravimetric analysis (TGA) and subsequent interaction of by-products with zirconia nanoparticles at different temperatures is investigated from FTIR spectra. Further the structure of nanoparticles annealed at different temperatures is investigated using XRD, micro Raman and photoluminescence (PL) analysis. Copyright © 2017 Inderscience Enterprises Ltd.

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2015

Journal Article

J. R, Valarmathi, A., and Dr. Sivakumar M., “Study on the Influence of Laser Beam Polarization on Micromachining of Crystalline Silicon Using Ultrafast Laser Pulses”, Lasers in Engineering, vol. 32, pp. 19-35, 2015.[Abstract]


Ultrafast laser machining is a unique technique to fabricate nano- and microscale structures with limited distortion to substrate materials. In the present work the influence of laser beam polarization direction on micromachining of crystalline Si with ultrafast laser pulses has been reported. The geometry of hole, ablation depth, machined features, kerf width and edge quality are studied with respect to the state of polariza-tion. In terms of hole geometry, machining with S-polarized beam gener-ates nearly circular holes as compared to asymmetrical stretching of the holes machined with P-polarization. Moreover, the depth of holes and channels machined with P-polarized beam is higher when compared to the depths of features machined with S-polarized beam. In multi-pulse, ultrafast laser machining, absorption of incident radiation is highly dependent on state of laser polarization and angle of incidence after first few pulses. The absorption is significantly higher for P-polarized beam resulting greater ablation depths. It is apparent from the experimental results that state of laser polarization is an important parameter in micro-machining of semiconductors with ultrafast laser pulses under ambient condition.

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2015

Journal Article

Dr. Sivakumar M. and Wang, J., “Controlled Fabrication of Micro/Nano-Structures on Germanium Using Ultrashort Laser Pulses under Ambient Conditions”, Advanced Materials Research, vol. 806, pp. 440-445, 2015.[Abstract]


A technique for ordered fabrication of periodic freestanding micro/nanostructures on the crystalline germanium (Ge) <100> surfaces with 1064 nm wavelength ultrashort laser pulses under ambient conditions is presented. The laser radiation fluence used for obtaining the structures is close to the melting threshold (0.1 J/cm2) of Ge. The dimensions of structures range from hundreds of nanometres to a few microns. The orientation of the periodic surface structures depends on laser beam polarization direction. Arrays of structures are formed in rows parallel to the sample movement direction for samples machined with s-polarized laser pulses, but formed in the direction perpendicular to the movement for p-polarized pulses. The structures are fabricated under variable temperatures on sample surface owing to the changed interference between incident and reflected laser beams. A micro-Raman analysis of the processed surfaces shows a minor change in the spectral intensity as compared to the unprocessed surface and the material retains its crystallinity after laser irradiation.

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2015

Journal Article

Dr. Umamaheswari A., Mohan, S. R., Dr. Sivakumar M., and S Kumar, S., “Tweaking Electrical and Dielectric Properties of Nickel Oxide Nanocrystals by Varying the Surfactant”, Journal of nanoscience and nanotechnology, vol. 15, pp. 9423–9430, 2015.[Abstract]


The influence of cationic cetyltrimethylammonium bromide (CTAB) and neutral polymeric polyvinylpyrrolidone (PVP) surfactants on electrical and dielectric properties of NiO nanocrystals is investigated. It is demonstrated that, compressive strain of nanocrystals is higher with PVP than that of CTAB. Consequently surfactant type has significant influence on intrinsic defects of nanocrystals. This is attributed to the difference in stabilization of metallic ions against agglomeration that leads to variation in rate of hydrolysis. Particularly, in the case of PVP assisted synthesis, higher stabilization leads to slow nucleation rate with lower defect density. As a result the hopping time of charge carriers decreases which in turn enhances the conductivity of nanocrystals as evidenced from the shifting of dielectric loss peak to higher frequency.

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2015

Journal Article

E. R. Mohan, Dr. Sivakumar M., and K.S. Sreelatha, “Modelling evanescent field components in metal-oxide core waveguides for nonlinear applications”, Materials Research Bulletin, vol. 69, pp. 131-137, 2015.[Abstract]


In a thin film planar geometry, the guiding effect due to the linear index results in light confinement in the thin film layer, at the same time causing evanescent waves to decay into the outer substrate or cladding layers. The magnitude of the field in the cladding is dependent on the characteristics of the waveguide and the wavelength of the wave. In this paper, we have modelled four metal-oxide core waveguides with prime focus on the evanescent field as light propagates through these optical structures. ZnO waveguide structure is taken as the benchmark. The evanescent path resembles a soliton consistent with the input soliton solution. A comparison is also carried out with a lower metal oxide core, MgO and a higher index TiO2. The variation in refractive index with input wavelength induces second order nonlinearity in these waveguide structures. For the simulated structures, mode field distributions are plotted against the refractive index. The magnitude of evanescent field also reduces with an increase in the input wavelength for the structures. The nature of the soliton path is insightful for the development of evanescent field based sensors. Optical waveguides with metal-oxide cores can act as a cheaper substitution to nanophotonic devices and integrated optics applications. With the study of evanescent phenomena in such structures, a minimally invasive manufacturing method without disturbance to the core may be made beneficial. © 2015 Elsevier Ltd. All rights reserved.

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2014

Journal Article

R. Elsa Mohan, Dr. Sivakumar M., and K.S. Sreelatha, “Comparison of soliton evolution in single-mode triangular-index thin film waveguides using different metal-oxide core with cladding air”, International Journal of Engineering and Innovative Technology (IJEIT), vol. 3, no. 10, pp. 281 - 285, 2014.[Abstract]


pplications to act as inter connectors, optical switches and routers. In the present study planar waveguides of nonlinear metal oxides MgO, TiO2 and ZnO as core material with a triangular refractive index profile are modeled for various input wavelengths. The variation in refractive index with input wavelength induces nonlinearity in these waveguide structures. A numerical continuation procedure is employed using an initial soliton solution for a triangular refractive index profile. An oscillatory wave path in the waveguide structures is seen to result. By doping an increase in refractive index of the core is achieved whereby oscillatory nature can be eliminated.

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2014

Journal Article

Dr. Umamaheswari A., Mohan, S. R., S. Kumar, S., and Dr. Sivakumar M., “Phase tuning of zirconia nanocrystals by varying the surfactant and alkaline mineralizer”, Ceramics International, vol. 40, pp. 6561 - 6568, 2014.[Abstract]


The influence of cationic (CTAB)/neutral polymeric (PVP) surfactants and strong (NaOH)/weak (NH4OH) alkaline mineralizers on phase stabilization of zirconia nanocrystals synthesized by chemical precipitation is investigated. X-ray diffraction and micro-Raman analysis of the as-prepared samples show that tetragonal zirconia is predominant as compared to monoclinic using \{PVP\} with NH4OH. The phases are also evident from lattice fringes of \{TEM\} images and the corresponding \{SAED\} pattern. Photoluminescence spectra of samples reveal oxygen vacancies present in the zirconia nanocrystals. The group H Raman vibration modes identified are attributed to surface defects and quantum size effects of nanocrystals. The phase stabilization of zirconia nanocrystals is explained using the polymerization rate of tetramers during synthesis. The rate can be varied by proper selection of the surfactant and the mineralizer. A slow polymerization rate with \{PVP\} and \{NH4OH\} favors the formation of tetragonal zirconia. Thus, a simple method for phase stabilization of zirconia nanocrystals at room temperature using chemical precipitation by varying the surfactant and the mineralizer is demonstrated.

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2013

Journal Article

U. Maheswari A, S Kumar, S., and Dr. Sivakumar M., “Influence of Alkaline Mineralizer on Structural and Optical Properties of ZrO2 Nanoparticles”, Journal of nanoscience and nanotechnology, vol. 13, pp. 4409-14, 2013.[Abstract]


In this work, the influence of different alkaline mineralizers on structural and optical properties of zirconia nanoparticles synthesized by chemical co-precipitation technique using zirconium oxychloride octahydrate (ZrOCI2 x 8H2O) as precursor is studied. The mineralizers used for the synthesis of nanoparticles are NaOH and NH4OH. X-ray diffraction, Fourier Transform Infrared (FTIR) spectroscopy, UV-visible absorption spectroscopy and Photoluminescence (PL) spectroscopy were used for characterizing the nanoparticles. Structural analysis of the sample synthesized using NaOH shows monoclinic phase as predominant one, however when NH4OH is used the major phases are cubic and tetragonal. The difference is attributed to the number of hydroxyl ions produced and their rate of release during the reaction process. The presence of these phases in both samples is further confirmed by vibrational bands of FT-IR spectra. Also, the low energy bands due to the presence of defects in nanoparticles are also explicitly observed in the photoluminescence spectra. Further the defects lead to a red shift in the band gap of ZrO2 which is observed when the samples are subjected to UV-absorption spectroscopy analysis. It has been demonstrated that zirconia nanoparticles with desired structural properties can be synthesized by changing the type of mineralizer without the necessity of doping.

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2013

Journal Article

Dr. Sivakumar M., Wang, J., and Huang, C. Z., “Laser-material interaction and grooving performance in ultrafast laser ablation of crystalline germanium under ambient conditions”, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, vol. 227, pp. 1714-1723, 2013.[Abstract]


Microgrooving on crystalline germanium (Ge) 〈100〉. surface using 1064 nm wavelength ultrafast laser pulses under ambient condition is investigated. The interaction of laser and target material and the influence of processing parameters such as laser power, pulse repetition rate and scan speed on the groove dimensions and surface roughness are studied. For the laser radiation fluence range used (0.4-0.8 J/cm2), material removal is primarily controlled by optical penetration depth. The depth and width of grooves increase with laser power. In multipulse irradiation, heat accumulation due to residual thermal energy from successive laser pulses results in a greater material removal. Furthermore, groove depth and width decrease as the pulse repetition rate increases from 0.5 to 2 MHz, due to the decrease in pulse energy with an increase in repetition rate causing ablation threshold fluence to move towards the central portion of the Gaussian pulse. Surface roughness has not shown significant changes for the parameters used in this study. A micro-Raman analysis of groove surfaces reveals a change in the crystallinity of the Ge due to laser irradiation. © IMechE 2013.

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2012

Journal Article

K. S. Sreelatha, Dr. Sivakumar M., and Mohan, R. Elsa, “Modeling of Complex Solitary Waveforms for Micro-Width Doped ZnO Waveguides”, International Journal of Modern Nonlinear Theory and Application, vol. 1, pp. 130-134, 2012.[Abstract]


The potential applications of metallic oxides as supporters of nonlinear phenomena are not novel. ZnO shows high nonlinearity in the range 600 - 1200 nm of the input wavelength [1]. ZnO thus make way to become efficient photoluminescent devices. In this paper, the above mentioned property of ZnO is harnessed as the primary material for the fabrication of waveguides. Invoking nonlinear phenomena can support intense nonlinear pulses which can be a boost to the field of communication. The modeling characteristics of undoped and doped ZnO also confirm the propagation of a solitary pulse [1]. An attempt to generalize the optical pattern of the doped case with varying waveguide widths is carried out in the current investigation. The variations below 6 um are seen to exhibit complex waveforms which resemble a continuum pulse. The input peak wavelength is kept constant at 600 nm for the modeling.

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2012

Journal Article

R. E. Mohan, Sreelatha, K. S., Dr. Sivakumar M., and Krishnashree, A., “Modeling of doped ZnO waveguides for nonlinear applications”, Advanced Materials Research, vol. 403-408, pp. 3753-3757, 2012.[Abstract]


In this paper, the modeling investigations of ZnO planar waveguides with varying refractive indices are quoted. The nature of field mode propagations highlights the nonlinear nature of ZnO. The field distributions of the doped structure are also considered and compared. We have focused on the 600-1200 nm wavelengths of the input beam, where nonlinear effects for ZnO are observed to be at their prime. It is observed that the mode distributions in the doped structure define solitary pulse propagation in itself. © (2012) Trans Tech Publications, Switzerland.

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2012

Journal Article

N. S. Roshima, Kumar, S. S., A.U, D. Umamaheswa, and Dr. Sivakumar M., “Study on vacancy related defects of CdS nanoparticles by heat treatment”, Journal of Nano Research, vol. 18-19, pp. 53-61, 2012.[Abstract]


In this work a method of incorporating anion or cation vacancy during synthesis stage of CdS nanoparticles to induce defect level emission is presented. Further the influence of temperature on this vacancy related defects is also studied. The as-prepared samples with co-precipitation technique were heat treated with different time intervals at a constant temperature of 200 °C. From UV-Visible absorption spectra, the band gap of both the as-prepared and heat treated samples are calculated to be 3.51 eV indicating that there are no significant changes in the size of nanoparticles. The photoluminescence spectra of both samples showed emission bands corresponding to band edge and defect levels. Further from the spectra, it was observed that the intensity of band edge luminescence decreases with increase of heat treatment duration. This is due to the fact that induced defects have reached the surface of nanoparticles. © (2012) Trans Tech Publications, Switzerland.

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2011

Journal Article

Dr. Sivakumar M., Venkatakrishnan, K., and Tan, B., “Characterization of mhz pulse repetition rate femtosecond laser-irradiated gold-coated silicon surfaces”, Nanoscale Research Letters, vol. 6, no. 1, pp. X1-5, 2011.[Abstract]


In this study, MHz pulse repetition rate femtosecond laser-irradiated gold-coated silicon surfaces under ambient condition were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), and X-ray photoelectron spectroscopy (XPS). The radiation fluence used was 0.5 J/cm2 at a pulse repetition rate of 25 MHz with 1 ms interaction time. SEM analysis of the irradiated surfaces showed self-assembled intermingled weblike nanofibrous structure in and around the laser-irradiated spots. Further TEM investigation on this nanostructure revealed that the nanofibrous structure is formed due to aggregation of Au-Si/ Si nanoparticles. The XRD peaks at 32.2°, 39.7°, and 62.5° were identified as (200), (211), and (321) reflections, respectively, corresponding to gold silicide. In addition, the observed chemical shift of Au 4f and Si 2p lines in XPS spectrum of the irradiated surface illustrated the presence of gold silicide at the irradiated surface. The generation of Si/Au-Si alloy fibrous nanoparticles aggregate is explained by the nucleation and subsequent condensation of vapor in the plasma plume during irradiation and expulsion of molten material due to high plasma pressure. © 2011 Sivakumar et al.

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2011

Journal Article

Dr. Sivakumar M., Tan, B., and Venkatakrishnan, K., “Characterization of high temperature conductive graphite surfaces irradiated with femtosecond laser pulses”, Applied Surface Science, vol. 257, pp. 9780-9784, 2011.[Abstract]


In this study high temperature conductive graphite surfaces irradiated with megahertz pulse repetition rate femtosecond laser pulses under ambient condition were characterized using electron microscopy and spectroscopy techniques. Scanning electron microscopy analysis of the treated surface shows formation of self assembled weblike nanofibrous structure in and around the laser irradiated spots. Further transmission electron microscopy investigation revealed that this structure was formed due to aggregation of graphite nanoparticles. In addition the broadening of microraman peaks at 1340 and 1580 cm -1 of the laser irradiated sample was due to confinement of optical phonons in graphite nanoparticles. X-ray photoelectron spectroscopy analysis shows a marginal increase of sp 2 and sp 3 species with laser treated samples as compared to that of untreated samples. The results show that femtosecond laser treatment is rather a simple technique for the direct synthesis graphite nanostructures without significant changes in their chemistry as compared to the bulk. © 2011 Elsevier B.V. All rights reserved.

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2011

Journal Article

Dr. Sivakumar M., Tan, B., and Venkatakrishnan, K., “Synthesis of TiO2 nanoscale rods with MHz femtosecond laser irradiation of single crystal surface and characterisation”, AIP Advances, vol. 1, 2011.[Abstract]


Growth of nanoscale rods on single crystal rutile TiO2 surface irradiated by MHz pulse repetition rate femtosecond laser in nitrogen environment without a catalyst or template is reported. The rods are of 100 nm in width to 1 micron length. Microraman analysis of the laser irradiated surface shows only a decrease in the intensity of active modes as compared to untreated surface. The growth of TiO2 nanorods can be explained by a method combining nanoparticles formation due to expulsion of molten material from laser irradiated spot and their subsequent growth by vapor-liquid-solid process.

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2010

Journal Article

Dr. Sivakumar M., Tan, B., and Venkatakrishnan, K., “Enhancement of Silicon Nanostructures Generation Using Dual Wavelength Double Pulse Femtosecond Laser Under Ambient Condition”, Journal of Applied Physics, vol. 107, p. 044307, 2010.[Abstract]


In this study we propose a unique method to increase the weblike silicon nanofibrous structures formation using dual wavelength double pulse femtosecond laser radiation under ambient condition. The augmentation of nanostructures is evidenced from the difference in nanofibrous structure layer thickness. Enhancement in generation is explained through the increase in excited state electrons with double pulse as compared to single pulse. Moreover the absorption characteristic of irradiated surface undergoes significant changes after the first pulse (515 nm) which enhances absorption for the second pulse (1030 nm) and consequently results in an increase in nanostructures. More »»

2010

Journal Article

Dr. Sivakumar M., Venkatakrishnan, K., and Tan, B., “Study of Metallic Fibrous Nanoparticle Aggregate Produced Using Femtosecond Laser Radiation Under Ambient Conditions”, Nanotechnology, vol. 21, p. 225601, 2010.[Abstract]


In this study, we report formation of weblike fibrous nanoparticle aggregate due to irradiation of bulk iron, aluminium and titanium samples using femtosecond laser radiation at MHz pulse repetition frequency in air at atmospheric pressure. Electron microscopy analysis revealed that the nanostructure is formed due to aggregation of polycrystalline nanoparticles of the respective constituent materials. The nanoparticle diameter varies between 5 and&nbsp;40&nbsp;nm and they are covered with an oxide layer of a few nanometres thick. X-ray diffraction and micro-Raman analysis revealed metallic and oxide phases in the nanostructure. The formation of a nanoparticle aggregate is explained by nucleation and condensation of vapour in the plasma plume and by phase explosion. Moreover the laser interaction time plays a significant role in the generation of nanostructure from bulk metals. This study provides evidence that femtosecond laser irradiation can be an ambient condition physical method for metallic fibrous nanoparticle aggregate generation.

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2009

Journal Article

Dr. Sivakumar M., Oliveira, V., Vilar, R., and Oliveira, S., “Shear Bond Stress of Composite Bonded to Excimer Laser Treated Dentin”, Journal of Laser Applications, vol. 21, pp. 129–132, 2009.[Abstract]


The aim of this work was to study the bond strength of resin composite bonded to dentin surfacestreated with KrF excimer laser radiation, untreated surfaces, and acid-etched surfaces using a single-plane shear method. Dentin specimens cut from freshly extracted permanent molar teeth were subjected to laser treatment with a KrF excimer laser (248 nm) using a fluence of 1 J/cm2. The bond strength was greater for acid-etched specimens than for laser treated or untreated specimens. The low strength of the bond to laser treatedsurfaces is probably due to a shift from a mixed to a cohesive rupture mechanism. It was concluded from this study that surface treatment of dentin surfaces with KrF excimer laser under the conditions described does not significantly improve the shear bond strength to composites. More »»

2006

Journal Article

Dr. Sivakumar M., Oliveira, V., Oliveira, S., Leitão, J., and Vilar, R., “Influence of Tubule Orientation on Cone-Shaped Texture Development in Laser-Ablated Dentin”, Lasers in medical science, vol. 21, pp. 160–164, 2006.[Abstract]


In the present paper, the influence of tubule orientation on surface texture development was studied. Specimens of dentin with a wide range of tubule orientations were extracted from caries-free human teeth, processed using KrF laser radiation, and analyzed by scanning electron microscopy. When a transverse cross section of dentin cut perpendicularly to the tooth axis is processed with KrF laser radiation, a cone-like topography develops in the inner dentin where tubules are parallel to the laser beam. When laser processing is carried out in the outer dentin, because tubules are significantly tilted with respect to the laser beam, flat surfaces are achieved. The surface texture after laser processing depends effectively on the angle between the tubules and the laser beam. The dependency of cone growth on tubule orientation was confirmed using a simple differential ablation model. More »»

Publication Type: Conference Paper

Year of Publication Publication Type Title

2008

Conference Paper

Dr. Sivakumar M., Oliveira, V., Vilar, R., and Rego, A. M. Botelho do, “KrF Excimer Laser Ablation of Human Enamel”, in Materials Science Forum, 2008.[Abstract]


Laser treatment is a promising technique for dental applications such as caries prevention, dental hypersensitivity reduction and improvement of bond strength of restoration materials. In this study the morphological, structural and chemical changes of enamel surface due to treatment with KrF excimer laser radiation were evaluated using scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. For radiation fluences near 1 J/cm², laser processing originates a relatively porous surface due to preferential removal of material in the enamel prism sheaths. Increasing the fluence leads to a relatively flat surface with clear evidence of surface melting. The X-ray diffractograms of both treated and untreated enamel are similar and correspond to hydroxyapatite. The only modification due to the laser treatment is a slight shift of the peaks, probably, due to a loss of the structural water of hydroxyapatite. X-ray photoelectron spectroscopy confirmed that organic matter is removed from the irradiated surface but no significant changes in the mineral phase occur. More »»