Qualification: 
Ph.D, MSc, BSc
s_ramasubramanian@cb.amrita.edu

Dr. S. Ramasubramanian currently serves as Assistant Professor in Physics, Department of Sciences, School of Engineering, Coimbatore Campus.

He obtained his master’s degree from Manonmaniam Sundaranar University (2007) and Doctoral degree from Crystal Growth Centre at Anna University (2014). His area of research includes semiconductor physics, p-type transparent semiconducting oxides and luminescence materials. He has been interest in defect analysis in semiconductors and role of dopants on the properties of semiconductors. He has also engaged on the theoretical investigations of different properties in materials using density functional theory. He has presented his research works in more than 15 national and international conferences.

Affiliation(s)

  • Assistant Professor, Department of Sciences, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, 2016 – Present
  • Teaching Fellow, Department of Physics, Anna University, Chennai, 2014-2016

Qualification

  • 2014: Ph.D.
    Crystal Growth Centre, Anna University, Chennai
  • 2007: M.Sc. Physics
    Department of Physics, Manonmaniam Sundaranar University, Tirunelveli, Tamilnadu
  • 2005: B.Sc. Physics
    Sri Paramakalyani College, Alwarkurichi, Tamilnadu

Awards, Certificates, Honors and Societies

  1. National Doctoral Fellow (NDF) - AICTE (2009)
  2. Received short term JSPS fellowship for visiting Nagoya Institute Technology, Nagoya, Japan – 2009 & 2011

Research interest

  • Area of Interest
    • Semiconductor Physics: Zinc Oxide nanorods based photodetectors, Dilute magnetic semiconductors
    • P-type Semiconductors: Copper oxide based transparent conducting oxide, Improving conductivity in other p-type semiconductors, transparent heterojunction devices
    • Luminescence materials: Upconversion phosphors, Phosphors for improving the solarcells efficiency
  • Keywords
    • ZnO
    • Dilute magnetic semiconductors
    • p-type semiconductors
    • photodetectors
    • Luminescence
  • Research Group
    Present Team

    Mr. Akshai Shyam is a Ph.D. student works in developing p-type transparent conducting oxide-based devices

Publications

Publication Type: Conference Proceedings

Year of Publication Title

2020

K. Janani, Dr. S. Ramasubramanian, and Thiyagarajan, P., “Tuning emission colour in Tm3+ doped Yb3+: KLnF4 (Ln = Y, Yb and Gd) upconversion phosphors”, Materials Today: Proceedings, vol. 33. pp. 2082-2085, 2020.[Abstract]


Yb3+-Tm3+ codoped KLnF4 (Ln = Y, Yb and Gd) phosphors have been synthesized by a solid-state reaction technique. The results revealed that KYF4 and KYbF4 phosphors crystallize in hexagonal structure and KGdF4 phosphor form in mixed phases. Prepared phosphors are subjected to photoluminescence studies under 980 nm and 356 nm wavelength excitation. The emission colour is examined using the CIE chromaticity diagram for the different host matrices. The results show that the developed phosphors could be useful in the field of lighting and emission colour tuning based applications.

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2012

M. Rajagopalan, Dr. S. Ramasubramanian, and Kumar, J., “First -principles calculations of ferromagnetism in ZnO co-doped with cobalt and nitrogen”, American Institute of Physics (AIP) Conference Proceedings, vol. 1447. pp. 1155-1156, 2012.

2012

M. Rajagopalan, Dr. S. Ramasubramanian, and Kumar, J., “Theoretical study of electronic and elastic properties of Li(Mg, Zn)N alloy”, AIP Conference Proceedings, vol. 1447. pp. 809-810, 2012.

Publication Type: Journal Article

Year of Publication Title

2020

M. Roshith, Pathak, A., Dr. A. K. Nanda Kumar, Anantharaj, G., Saranyan, V., Dr. S. Ramasubramanian, Babu, T. G. Satheesh, and Kumar, D. V. Ravi, “Continuous flow solar photocatalytic disinfection of E. coli using red phosphorus immobilized capillaries as optofluidic reactors”, Applied Surface Science, vol. 540, p. 148398, 2020.[Abstract]


An elemental, non-metallic red phosphorus-based photocatalyst for potential continuous flow disinfection of water is reported. The crystalline red phosphorus is immobilized by a solid state method on the inner walls of a quartz capillary tube and a continuous flow photocatalytic disinfection of E. coli solution under direct sunlight is demonstrated using the set-up as an optofluidic reactor. Structural and microstructural analyses employing electron diffraction confirms the fibrous phase of the immobilized red phosphorus. The reactor with the immobilized photocatalyst when tested under direct sunlight resulted in a 6.7 log reduction (>99.9999% reduction) in the concentration of E. coli bacteria within 14 min. The sample aliquot collected at 28 min residence time did not yield any visible colonies indicating the high efficiency of the process. The demonstrated efficiency suggests great potential for commercial scale-up.

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2019

N. V, K, J., Darba, V. Ravikumar, and Dr. S. Ramasubramanian, “Investigations on the synthesis dependent luminescence property of LiGdF4: Eu3+ microcrystals”, Optik, vol. 198, p. 163233, 2019.[Abstract]


Lanthanide ion doped inorganic crystals have attracted appreciable interest due to their distinct properties such as high luminescence efficiency, photo-chemical stability which explored their commercialization in display and illumination devices. In this work, LiGdF4 has chosen as a host for doping Eu3+ as it posses low phonon energy and chemical stability. Initially, LiGdF4: Eu phosphor is synthesized using modified solid state reaction method with varying temperature from 500°C to 800°C. X-ray diffraction analysis showed the synthesized microcrystals are in tetragonal structure with observation of impure phase at lower temperature which disappeared as the temperature increased. Ball milling of the sample synthesized at 600 °C has been performed at different hours. Photoluminescence of the samples are analyzed with UV excitation of 273 nm. Sharp green luminescence is observed for all samples and the intensity is found to get decreased with reduction of crystallinity of the samples during ball milling.

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2019

M. Roshith, Kumar, M. S., Kumar, A. K. Nanda, Dr. S. Ramasubramanian, Stanley, J., Dr. Satheesh Babu T. G., and Kumar, D. V. Ravi, “Urchin-like fibrous red phosphorus as an efficient photocatalyst for solar-light-driven disinfection of E. coli”, Journal of Photochemistry and Photobiology A: Chemistry, vol. 384, 2019.[Abstract]


Fibrous red phosphorus (FRP), a crystalline allotrope of red phosphorus has interesting structural features and promising applications in the area of energy storage devices and photocatalysis. In this work, we employ a solid state method to synthesize crystalline red phosphorus which results in an interesting urchin-like structure. The microstructure shows a bundle of fibers originating from a core. Detailed characterization points to the presence of FRP as the major allotrope in the product. Photocatalytic activity of FRP is examined towards the solar disinfection of water using E. coli as a model pollutant. The bandgap of the synthesized FRP is 1.9 eV and it has optimum valence band and conduction band levels to generate reactive oxygen species (ROS) such as superoxide radical and H2O2. OH· is then produced from the H2O2. These photo-generated ROS are successful in bringing 8 log reduction in the concentration of E. coli in 30 min, which compares extremely well with values reported from other catalysts. © 2019 Elsevier B.V.

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2019

K. Janani, Dr. S. Ramasubramanian, Thangavel, R., and Thiyagarajan, P., “Effect of gadolinium concentration on the luminescence of LiYF4:Yb3+/Er3+ phosphor”, Solid State Sciences, vol. 91, pp. 119-125, 2019.[Abstract]


Tetragonal phase of LiY1-xGdxF4 codoped with Yb/Er phosphors has been synthesized via solid-state reaction process for the optical behavior. X-ray photoelectron spectroscopy analysis has confirmed the presence of dopants and gadolinium in the expected charge state in the host lattice. Upconverted green and red emissions have been observed for the synthesized fluoride LiY1-xGdxF4 phosphors at 980 nm excitation. Investigations are made to optimize the upconverted emission intensities and colour by changing gadolinium concentration in the host matrix. The photoluminescence and decay curve analysis has been carried out at 378 nm wavelength excitation in the developed phosphors.

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2018

K. Janani, Dr. S. Ramasubramanian, Soni, A. K., Rai, V. K., and Thiyagarajan, P., “Luminescence properties of LiYF4:Yb3+, Er3+ phosphors: A study on influence of synthesis temperature and dopant concentration”, Optik, vol. 169, pp. 147-155, 2018.[Abstract]


Yb3+ and Er3+ doped LiYF4:RE3+ (RE = Yb, Er) phosphors have been synthesized using solid state reaction of pre-converted rare earth fluorides and lithium fluoride. Structural and morphological properties of the synthesized fluoride phosphors are examined. Upconversion emission studies performed in the developed phosphors showed blue, green and red emission bands under 980 nm excitation. Effect of Yb3+ ions concentration and synthesis temperature on the upconversion emission bands has been investigated. As the synthesis temperature increased the phosphors exhibited intense upconversion luminescence. The colour co-ordinate analysis confirmed the strong green emission under upconversion process. Luminescence in synthesized phosphors have also been investigated with respect to 378 nm excitation. The prepared phosphors have shown good luminescent properties and could be a potential candidate for luminescent based device fabrications. © 2018 Elsevier GmbH

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2017

S. N. Kaleemullah, Dr. S. Ramasubramanian, Mohankumar, R., S. Basha, M., Rajagopalan, M., and Kumar, J., “Magnetic properties of gadolinium and carbon co-doped gallium nitride”, Solid State Communications, vol. 249, pp. 7-11, 2017.[Abstract]


Investigations have been carried out to study the ferromagnetic properties of Gadolinium (Gd) Carbon (C) co-doped wurtzite Gallium Nitride (GaN) using full-potential linear augmented plane wave (FP-LAPW) method within the density functional theory. The system shows half-metallic nature when single Gd is substituted in Ga36N36 supercell. The presence of carbon in GaN supercell is found to generate weak magnetic moment (Ms) in the neighbouring atoms. When Carbon is codoped in the Gd-GaN, it increased the total magnetic moment of the system (Mtot). The cause of ferromagnetism in the Gd and C co-doped GaN has been explained by Zener’s p-d exchange mechanism. The role of defects in the magnetic property of this system is also investigated. The results indicate the gallium vacancy influences the magnetic moment of the Gd and C codoped GaN more than the nitrogen vacancy. The presence of holes is effective than electrons in achieving the ferromagnetism in the considered system.

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2015

R. Mohankumar, Dr. S. Ramasubramanian, Rajagopalan, M., M. Raja, M., Kamat, S. V., and Kumar, J., “Density functional study of half-metallic property on B2 disordered Co2FeSi”, Journal of Materials science, vol. 50, no. 3, pp. 1287 - 1294, 2015.[Abstract]


First principle calculations on the magnetic properties and spin polarization of the B2 disordered Co2FeSi full Heusler alloy have been carried out along with that of ordered L21 structure. The linear augmented plane wave method, within the LDA+U approximation, gives the magnetic moment value as predicted by Slater-Pauling rule. A small band gap in the minority band structure around the vicinity of the Fermi level results in half-metallicity or 100 % spin polarization in L21 ordered structure. The calculations also showed that the half-metallicity vanishes at moderate percentages of B2 disorders due to creation of new states around the Fermi level in the minority DOS but at low and high levels of disorder, the half-metallicity was retained. The variation in the magnetic moments of Co2FeSi with change in the % disorder was attributed to the changes in the local magnetic moments of Co, Fe, and Si atoms which vary with change in % disorder.

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2015

R. Mohankumar, Dr. S. Ramasubramanian, Rajagopalan, M., M. Manivel Raja, Kamat, S. V., and Kumar, J., “Effect of Fe substitution on the electronic structure, magnetic and thermoelectric properties of Co2FeSi full Heusler alloy: A first principle study”, Computational Materials Science, vol. 109, pp. 34-40, 2015.[Abstract]


A large number of Heusler compounds have been discovered and their properties have been investigated both theoretically and experimentally. Among all the discovered Heusler compounds, Co2FeSi is focused for spintronic applications due to its high magnetic moment and high Curie temperature. In this aspect, the electronic structure, magnetic and thermoelectric properties of full Heusler alloy Co2−xFe1+xSi (x=0–1) have been investigated by using first principle, Full Potential Linear Augmented Plane Wave method. The density of states calculation shows half-metallicity at x=0 and 0.25 (i.e. Co2FeSi and Co1.75Fe1.25Si). The calculated magnetic moments were compared with experimental values measured at 4K and also with the magnetic moments calculated by Slater–Pauling rule. The thermoelectric properties of Co2FeSi and Fe2CoSi were also calculated over a wide range of temperatures and it was found that Fe2CoSi shows higher figure of merit than Co2FeSi.

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2013

Dr. S. Ramasubramanian, Thangavel, R., Rajagopalan, M., Thamizhavel, A., Asokan, K., Kanjilal, D., and Kumar, J., “Study on the ferromagnetism in Co and N doped ZnO thin films”, Current Applied Physics, vol. 13, pp. 1547-1553, 2013.[Abstract]


Present investigation reports the structural, optical and magnetic properties of co-doping of Co and N ions in ZnO samples, prepared by two distinct methods. In the first method, samples are synthesized by Sol–gel technique in which the Co and N are co-doped simultaneously during the growth process itself. In the second case, N ions are implanted in the Co doped ZnO thin films grown by Pulsed Laser Deposition (PLD). Structural studies showed that the nitrogen implantation on Co doped ZnO samples developed compressive stress in the films. X-ray photoelectron spectroscopy confirmed the doping of Co and N in ZnO matrix. In the Resonant Raman scattering multiple LO phonons up to fifth order are observed in the (Co, N) co-doped ZnO. Photoluminescence spectra showed that there is reduction in the bandgap due to the presence of Co in the lattice and also the presence of Zn vacancies in the films. All samples showed ferromagnetic behavior at room temperature. The magnetic moment observed in the implanted films is found to be varied with the different dosages of the implanted N ions. First principle calculations have been carried out to study the possible magnetic interaction in the co-doped system. Present study shows that the ferromagnetic interaction is due to the hybridization between N 2p and Co 3d states in the (Co, N) co-doped ZnO and is very sensitive to the geometrical configurations of dopants and the vacancy in the ZnO host lattice.

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2012

M. S. Basha, Dr. S. Ramasubramanian, Rajagopalan, M., and Kumar, J., “Investigations of cobalt and carbon codoping in gallium nitride for spintronic applications”, Journal of Magnetism and Magnetic Materials, vol. 324, pp. 1528-1533, 2012.[Abstract]


Extensive theoretical investigations have been carried out to study the ferromagnetic properties of transition metal doped wurtzite GaN using the Tight Binding Linear Muffin-tin Orbital (TBLMTO) method within the density functional theory. The present calculation reveals ferromagnetism in cobalt doped GaN when one gallium is replaced by cobalt in a 3×3×2 supercell of GaN, which gives rise to a cobalt concentration of 2.77%. The system is half-metallic with a magnetic moment of 4.0μB. When Co is bonded with one carbon, there is a drastic decrease in magnetic moment and the system becomes metallic. When Co dimer is introduced via nitrogen which corresponds to the Co concentration of 5.5% the magnetic moment is 3.99μB and the system is half-metallic. Same trend is observed when Co is bonded via nitrogen with unequal distance. When cobalt dimer is formed via carbon, the moment becomes 2.95μB and it shows metallic character. For dimer via carbon with unequal distance, the moment is 3.0μB and the system becomes semiconductor. For higher percentage of cobalt dopant the system shows metallic character. C and Co doped GaN samples have been synthesized experimentally and characterized with X-ray diffraction, transmission electron microscopy, micro-Raman and superconducting quantum interface device measurements. The observed results are correlated with the theoretical studies.

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2011

M. S. Basha, Dr. S. Ramasubramanian, Rajagopalan, M., Kumar, J., Kang, T. Won, N. Subramaniam, G., and Kwon, Y., “Investigations on cobalt doped GaN for spintronic applications”, Journal of Crystal Growth, vol. 318, pp. 432-435, 2011.[Abstract]


(Ga1−xCox)N was synthesized at 1223K using pure gallium and cobalt metal as precursors in a reactor specially designed for this purpose with ammonia as the nitrogen source. The structural confirmation of wurtzite phase of GaN is done using powder X-ray diffractometer. Photoluminescence shows red shift in band edge emission as the cobalt concentration increases from 1 to 5 atomic percentage together with nitrogen related vacancy emission at 3.24eV and cobalt related emission at 3.13eV. The magnetic properties were studied at 10K using superconducting quantum interface device (SQUID). The obtained maximum magnetic moments for 1%, 3% and 5% cobalt doped GaN are 1.63, 0.21 and 0.14μB, respectively. The decrease in magnetic moment with increase in dopant concentration may be due to the formation of secondary phases or cobalt cluster, which contributes to anti-ferromagnetism. From temperature dependent magnetization measurements it is observed that there is no drastic change in the magnetic moment upto room temperature for 1% cobalt doped GaN. Theoretical calculations based on the density functional theory within Tight Binding Linear Muffin-tin Orbital (TBLMTO) method has been carried out to study the magnetic properties of cobalt doped GaN. The obtained magnetic moment values per Co atom are 2.56μB for 1.56% and 2.77% and 1.50μB for 4.20%. This is found to be higher when compared with experimental values and this difference is explained based on structural defects and percentage of dopant used during synthesis. The half metallic behavior is observed for 1.56% and 2.77% Co dopants, but for higher percentage of dopant the system becomes metallic. Hence, it is concluded that only lower percentage of cobalt doped GaN is suitable for spintronic applications.

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2010

M. S. Basha, Dr. S. Ramasubramanian, Thangavel, R., Rajagopalan, M., and Kumar, J., “Magnetic properties of Ni doped gallium nitride with vacancy induced defect”, Journal of Magnetism and Magnetic Materials, vol. 322, pp. 238-241, 2010.[Abstract]


Investigations have been carried out to study the ferromagnetic properties of transition metal (TM) doped wurtzite GaN from first principle calculations using tight binding linear muffin-tin orbital (TBLMTO) method within the density functional theory. The present calculation reveals ferromagnetism in nickel doped GaN with a magnetic moment of 1.13μB for 6.25% of Ni doping and 1.32μB for 12.5% of nickel doping, there is a decrease of magnetic moment when two Ni atoms are bonded via nitrogen atom. The Ga vacancy (VGa) induced defect shows ferromagnetic state. Here the magnetic moment arises due to the tetrahedral bonding of three N atoms with the vacancy which is at a distance of 3.689Å and the other N atom which is at a distance of 3.678Å .On the other hand the defect induced by N vacancy (VN) has no effect on magnetic moment and the system shows metallic character. When Ni is introduced into a Ga vacancy (VGa) site, charge transfer occur from the Ni ‘d’ like band to acceptor level of VGa and formed a strong Ni–N bond. In this Ni–VGa complex with an Ni ion and a Ga defect, the magnetic moment due to N atom is 0.299μB .In case of Ni substitution in Ga site with N vacancy, the system is ferromagnetic with a magnetic moment of 1μB.

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2010

M. Sundareswari, Dr. S. Ramasubramanian, and Rajagopalan, M., “Elastic and thermodynamical properties of A15 Nb3X (X =  Al, Ga, In, Sn and Sb) compounds — First principles DFT study”, Solid State Communications, vol. 150, pp. 2057-2060, 2010.[Abstract]


Elastic and thermodynamical properties of Nb3X (X = Al, Ga, In, Sn and Sb) compounds are obtained by performing the band structure calculations, using the FP-LAPW method. The calculated lattice constants, elastic constants, Bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, Debye temperature and anisotropy ratio are reported and the results agree with the available data. From the present study, it is observed that stiffness and shear resistance are found to be greater in Nb3Sb than in other compounds. It is more brittle in nature and comparatively harder than other materials under study. The degree of anisotropy decreases as one moves across Nb3Ga, Nb3Al, Nb3In, Nb3Sb and Nb3Sn. From the calculated values of the Debye temperature, it is observed that thermal conductivity is greater for Nb3Sb and less for Nb3In.

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2009

Dr. S. Ramasubramanian, Thangavel, R., Rajagopalan, M., and Kumar, J., “Theoretical investigation of the electronic and optical properties of Zn2OX (X=S, Se, Te) in chalcopyrite phase by full potential methods”, Journal of Alloys and Compounds, vol. 479, pp. 414-419, 2009.[Abstract]


A full potential linear muffin tin orbital (FP-LMTO) and linear augmented plane wave plus local orbital (APW+lo) approach is used to investigate the electronic and optical properties of Zn2OX (X=S, Se, Te) in the chalcopyrite phase. It is found that these compounds are direct band semiconductors and the value of band gap increases as one move from Te to S. The optical properties of these compounds namely the reflectivity, refractive index, extinction coefficient, absorption coefficient, real part of optical conductivity and electron energy loss are calculated and their spectra are analyzed. In addition, the band structure, density of states and the bowing parameter of the Zn2OX (X=S, Se, Te) semiconductors are also obtained and given. The trends in physical properties are also discussed and compared with the available results.

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2009

Dr. S. Ramasubramanian, Mathrubutham, R., Thangavel, D. Rajalingam, and Kumar, J., “Ab initio study on elastic and thermodynamical properties of Ti 1-x Zr x C”, The European Physical Journal B - Condensed Matter and Complex Systems, vol. 69, pp. 265-268, 2009.

2009

Dr. S. Ramasubramanian, Mathrubutham, R., Kumar, J., and Thangavel, D. Rajalingam, “Electronic and optical properties of mixed anion layered oxychalcogenide semiconductors: An ab initio study”, Journal of Applied Physics, vol. 106, pp. 023720 - 023720, 2009.

Publication Type: Book Chapter

Year of Publication Title

2014

J. Kumar, Dr. S. Ramasubramanian, Thangavel, R., and Rajagopalan, M., “Chapter 15 - On the Optical and Magnetic Properties of Doped-ZnO”, in ZnO Nanocrystals and Allied Materials - Springer series in materials science, Springer 2014th ed., vol. 180, 2014, pp. 309-329.

Faculty Research Interest: