RAJKUMAR
@americancollege.edu.in
PHYSICS
THE AMERICAN COLLEGE
EDUCATION
Ph. D. Physics Madurai Kamaraj university, Madurai - 2011
M. Sc. Physics Madurai Kamaraj university, Madurai - 2006
B. Sc. Physics Vivekananda College, Madurai - 2004
RESEARCH INTERESTS
Synthesis and Characterization of Metal Oxide Semiconductor
Nanomaterials, Mataloxide, Raman Spectroscopy, Diluted Magnetic Semiconductors (DMS),
Gas Sensors, and Dye-Sensitized Solar Cells (DSSC)
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
S. Maheswari, M. Karunakaran, L. Bruno Chandrasekar, K. Kasirajan, and N. Rajkumar
Springer Science and Business Media LLC
The original version of the article (https://doi.org/10.1007/s10854-020-03809-6) was unfortunately published with errors in Table 2 and Fig. 9a. These errors are corrected in this article. The corrected table and figure are given below.
S. Maheswari, M. Karunakaran, L. Bruno Chandrasekar, K. Kasirajan, and N. Rajkumar
Springer Science and Business Media LLC
Nd-doped SnO2 thin films are prepared by the nebulizer spray pyrolysis method. The compositional and morphological studies are discussed. The X-ray diffraction reveals that the films are polycrystalline in nature. The grain size increases as the doping concentration of Nd increases. The x-axis orientation of the films is enhanced by Nd doping. The intensity of the miller indices (200) is enhanced due to doping. In the Raman spectrum, the doping concentration-dependent intensity is observed. The quenching is observed in the photoluminescence spectrum. The transmittance and the band gap of films have been decreased due to doping. The 5 wt% Nd-doped film shows the maximum response to ammonia.
Venkatraman Ravi Kumar, Nagappan Rajkumar, Freek Ariese, and Siva Umapathy
American Chemical Society (ACS)
The photochemistry of aromatic ketones plays a key role in various physicochemical and biological processes, and solvent polarity can be used to tune their triplet state properties. Therefore, a comprehensive analysis of the conformational structure and the solvent polarity induced energy level reordering of the two lowest triplet states of 9,10-phenanthrenequinone (PQ) was carried out using nanosecond-time-resolved absorption (ns-TRA), time-resolved resonance Raman (TR(3)) spectroscopy, and time dependent-density functional theory (TD-DFT) studies. The ns-TRA of PQ in acetonitrile displays two bands in the visible range, and these two bands decay with similar lifetime at least at longer time scales (μs). Interestingly, TR(3) spectra of these two bands indicate that the kinetics are different at shorter time scales (ns), while at longer time scales they followed the kinetics of ns-TRA spectra. Therefore, we report a real-time observation of the thermal equilibrium between the two lowest triplet excited states of PQ, assigned to nπ* and ππ* of which the ππ* triplet state is formed first through intersystem crossing. Despite the fact that these two states are energetically close and have a similar conformational structure supported by TD-DFT studies, the slow internal conversion (∼2 ns) between the T(2)(1(3)nπ*) and T(1)(1(3)ππ*) triplet states indicates a barrier. Insights from the singlet excited states of PQ in protic solvents [ J. Chem. Phys. 2015 , 142 , 24305 ] suggest that the lowest nπ* and ππ* triplet states should undergo hydrogen bond weakening and strengthening, respectively, relative to the ground state, and these mechanisms are substantiated by TD-DFT calculations. We also hypothesize that the different hydrogen bonding mechanisms exhibited by the two lowest singlet and triplet excited states of PQ could influence its ISC mechanism.
Venkatraman Ravi Kumar, Nagappan Rajkumar, and Siva Umapathy
AIP Publishing
Solvent effects play a vital role in various chemical, physical, and biological processes. To gain a fundamental understanding of the solute-solvent interactions and their implications on the energy level re-ordering and structure, UV-VIS absorption, resonance Raman spectroscopic, and density functional theory calculation studies on 9,10-phenanthrenequinone (PQ) in different solvents of diverse solvent polarity has been carried out. The solvatochromic analysis of the absorption spectra of PQ in protic dipolar solvents suggests that the longest (1n-π(1)*; S1 state) and the shorter (1π-π(1)*; S2 state) wavelength band undergoes a hypsochromic and bathochromic shift due to intermolecular hydrogen bond weakening and strengthening, respectively. It also indicates that hydrogen bonding plays a major role in the differential solvation of the S2 state relative to the ground state. Raman excitation profiles of PQ (400-1800 cm(-1)) in various solvents followed their corresponding absorption spectra therefore the enhancements on resonant excitation are from single-state rather than mixed states. The hyperchromism of the longer wavelength band is attributed to intensity borrowing from the nearby allowed electronic transition through vibronic coupling. Computational calculation with C2ν symmetry constraint on the S2 state resulted in an imaginary frequency along the low-frequency out-of-plane torsional modes involving the C=O site and therefore, we hypothesize that this mode could be involved in the vibronic coupling.
Rishikesh Pandey, Nagappan Rajkumar, and Siva Umapathy
Wiley
Substitution plays an important role in determining the triplet state reactivity. In this paper, we have studied the effect of chlorine substitution on the triplet state structure and the reactivity of thioxanthone (TX). We have employed time-resolved resonance Raman technique to understand the structure of the lowest triplet excited state of 2-chlorothioxanthone (CTX). The experimental findings have been corroborated with the computational results using density functional theory. Akin to the parent compound (TX), coexistence of two lowest triplet states has been observed in case of CTX, which has been substantiated using resonant probe wavelength dependence study. The relative contribution of 3n–π* to 3π–π* to the equilibrated triplet state has been found to be more for CTX compared to TX suggesting increase in the triplet state reactivity after the substitution. The above observation has been further supported by the flash photolysis experiments. Copyright © 2013 John Wiley & Sons, Ltd.
P. Indra Devi, N. Rajkumar, K. Ramachandran, Alka B. Garg, R. Mittal, and R. Mukhopadhyay
AIP
Room temperature ferromagnetism and an increase in saturation magnetization due to Mn‐doping are observed in CoFe 2 O 4 nanoparticles. The ethanol sensitivity of undoped and Mn‐doped (3 at.%) CoFe 2 O 4 nanoparticles were experimented at ambient temperature using optical fiber based on evanescent wave adsorption phenomenon. By modifying the clad exposure to gas vapor, the sensitivities were calculated to be 0.07 and 0.12 counts/ppm for undoped and Mn‐doped CoFe 2 O 4 nanoparticles respectively, showing significant enhancement in ethanol sensitivity for Mn‐doped sample.
N. Rajkumar, K. Ramachandran, Alka B. Garg, R. Mittal, and R. Mukhopadhyay
AIP
In this work, we report the structural, optical, and magnetic properties of Zn1−2xMnxCoxO (x = 0.00, 0.01, 0.02, and 0.03) nanostructures which were synthesized by seed based chemical method under ambient conditions. The X‐Ray diffraction analysis reveals the wurtzite structure of ZnO nanostructures with no secondary crystalline phase due to doping. The morphologies of samples were examined using scanning electron microscopy and transmission electron microscopy, which indicate the formation of ZnO nanorods (diameter∼150 nm and length∼10 μm) for undoped sample and the growth of nanorods were affected with the increase of dopant concentration. The elemental compositions were analyzed by energy dispersive spectroscopy and X‐Ray photoelectron spectroscopy (XPS), which infers the presence of oxygen deficiency due to rapid growth of nanostructures. From XPS analysis, the presence of Co and Mn in divalent form was confirmed and no metallic clusters in the semiconducting matrix. The absorption measurement shows t...
P. Indra Devi, N. Rajkumar, B. Renganathan, D. Sastikumar, and K. Ramachandran
Institute of Electrical and Electronics Engineers (IEEE)
Undoped and Mn-doped cobalt ferrite (CoFe2O4) nanoparticles were synthesized and characterized for thermal conductivity and magnetic properties. Room temperature ferromagnetism and an increase in saturation magnetization due to Mn doping (65.4 emu/g for 3 at.% of Mn and 20.8 emu/g for undoped CoFe2O4 nanoparticles) are observed. The ethanol gas sensitivity of undoped and Mn-doped (3 at.%) CoFe2O4 nanoparticles were experimented at ambient temperature using optical fiber based on clad-modified method. By modifying the clad exposure to gas vapor, the sensitivities were estimated to be 7 and 12 counts/100 ppm for undoped and Mn-doped CoFe2O4 nanoparticles, respectively, showing significant enhancement in ethanol gas sensitivity.
N. Rajkumar and K. Ramachandran
Institute of Electrical and Electronics Engineers (IEEE)
The compositional analysis done on the synthesized undoped and cobalt (Co)-doped titanium dioxide ( TiO2) nanoparticles by X-ray photoelectron spectroscopy and energy dispersive X-ray analysis suggests oxygen deficiency in undoped and Co-doped TiO2 samples. Magnetic investigations by vibrating sample magnetometer (VSM) indicate that undoped TiO2 nanoparticles possess ferromagnetic ordering at room temperature due to the oxygen deficiency and in Co-doped TiO2, the enhancement in magnetic moment is due to Co doping.
S. S. KANMANI, N. RAJKUMAR, and K. RAMACHANDRAN
World Scientific Pub Co Pte Lt
TiO 2 nanoparticles and TiO 2: ZnO (80:20) nanocomposites were prepared by chemical method, and X-ray diffraction, high resolution transmission electron microscopy, and Raman analysis of the samples showed a strong anatase phase and they are further used to find and compare the particle size.
N. RAJKUMAR, M. PRABHU, and K. RAMACHANDRAN
World Scientific Pub Co Pte Lt
A novel one-step method to synthesize large-scale uniform dumbbell-shaped ZnO nanorods is presented here. The structural morphology was investigated by using X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy analyses. The prepared dumbbells with diameter of 300 nm and length of 1.5 μm possess hexagonal structure with preferable [001] growth direction. The UV–vis absorption spectra show blueshift for smaller nanoparticles and redshift for bigger dumbbell-shaped ZnO nanorods.
N. Rajkumar, S. S. Kanmani, and K. Ramachandran
American Scientific Publishers
N. Rajkumar, V.M. Susila, and K. Ramachandran
Informa UK Limited
Nanocrystalline samples of Cd1− x Mn x O (x = 0.00, 0.02, 0.04 and 0.06) were synthesised by sol–gel method and characterised by X-ray diffraction, Faraday's rotation, Guoy's method and electron paramagnetic resonance (EPR) at room temperature (RT). Results reveal that ferromagnetism at RT for Mn-doped CdO nanoparticles is possible.
N. RAJKUMAR and K. RAMACHANDRAN
World Scientific Pub Co Pte Lt
Undoped and Mn (1 and 3 at.%) doped ZnO nanoparticles were synthesized by a simple chemical co-precipitation method. Rietveld refinement of X-ray diffraction (XRD) data revealed that undoped and Mn doped ZnO nanoparticles crystallize in the monophasic wurtzite structure and monotonous expansion of the lattice constants with increasing Mn content, due to the effective Mn doping. TEM images of all the samples showed the monodispersive spherical particles with the size of ~7 nm. The quantum confinement of nanoparticles was tested from UV–Vis absorbance measurement and the particle sizes were calculated and compared with TEM and XRD. The chemical compositions were analyzed by energy dispersive spectroscopy (EDS). In order to investigate the origin of ferromagnetism, the electronic structures of the Zn , O , and Mn atoms were probed by X-ray photoelectron spectroscopy (XPS). XPS data revealed that most of the dopants (Mn) exists in +2 oxidation state for 3 at.% Mn doped ZnO sample. The magnetization curves of the Mn doped ZnO samples indicate the existence of room-temperature ferromagnetic (RTFM) behavior. Here the observed RTFM in Mn doped ZnO can be attributed to the substitutional incorporation of Mn at Zn sites rather than due to the formation of any secondary phases.
N. RAJKUMAR, D. UMAMAHAESWARI, and K. RAMACHANDRAN
World Scientific Pub Co Pte Lt
Fe3O4 nanoparticles of size 10 and 12 nm were synthesized by chemical reduction method and characterized for their structural, optical, thermal and magnetic properties at room temperature. Photoacoustic analysis shows a reduction in thermal conductivity atleast by one order from the bulk but within the nanoregime, thermal conductivity increases with decreasing particle size. VSM measurements indicate superparamagnetism in Fe3O4 nanoparticles.
N. RAJKUMAR, R. N. MARIAMMAL, and K. RAMACHANDRAN
World Scientific Pub Co Pte Lt
Flower-type ZnO nanorods were synthesized by two-step low temperature solution growth and dye-sensitized with phenosafranine for the first time. The scanning electron microscope result shows that the diameter and the length of a single rod of the flower-type nanostructures depend on the method of synthesis. Optical absorption analysis shows a visible absorption at 517 nm, which was otherwise absent in nano ZnO . The photoluminescence spectra of ZnO and dye-sensitized ZnO nano flowers were also analyzed.
N. RAJKUMAR and K. RAMACHANDRAN
World Scientific Pub Co Pte Lt
Ferromagnetism (FM) at room temperature in nano ZnO:Mn is closely monitored here for very low concentration of Mn , as there are conflicting reports on FM, in the uniformly doped system.
P. VINOTHA BOORANA LAKSHMI, B. RANGANATHAN, N. RAJKUMAR, and K. RAMACHANDRAN
World Scientific Pub Co Pte Lt
Nanocrystalline samples of Zn 1-x Mn x O (x = 0.0, 0.01, 0.02, 0.1 and 0.2) were synthesized by solid state reaction and characterized by X-ray diffraction and electron paramagnetic resonance (EPR) at room temperature. EPR studies showed an evidence of ferromagnetism above room temperature.
P. Vinotha Boorana Lakshmi, N. Rajkumar, B. Ranganathan, and K. Ramachandran
Inderscience Publishers
Titanium dioxide (TiO2) nanocrystals are synthesised by chemical route and characterised for structural and optical properties. The heat transfer in the nanoparticles is studied from the thermal properties of the nanoparticles by photoacoustic spectroscopy (PA).
RECENT SCHOLAR PUBLICATIONS
MOST CITED SCHOLAR PUBLICATIONS
Publications
Articles Published in International Journals:
17. Room temperature ammonia gas sensor using Nd-doped SnO2 thin films and itscharacterization, S. Maheswari, M. Karunakaran, L. Bruno Chandrasekar, K. Kasirajan and N.Rajkumar,Journal of Materials Science: Materials in Electronics, 31, 12586 (2020)
16. Direct Observation of Thermal Equilibrium of Excited Triplet States of 9,10,Phenanthrenequinone. A Time-Resolved Resonance Raman Study
V. Ravikumar, N. Rajkumar, F. Ariese, and S. Umapathy,Journal of Physical Chemistry A, 119, 10147 (2015)
15. Solvatochromism of 9,10- phenanthrenequinone: An electronic and resonance Raman spectroscopic study,V. Ravikumar, N. Rajkumar, and S. Umapathy
The Journal of Chemical Physics, 142, 24305 (2015)
14. Effect of chlorine substitution on triplet state structure of thioxanthone: A time-resolved resonance Raman study, R. Pandey, N. Rajkumar and S. Umapathy,J. Raman Spectroscopy, 44, 270–276 (2013)
13. Oxygen deficiency and room temperature ferromagnetism in undoped and cobalt
doped TiO 2 nanoparticles
N. Rajkumar and K. Ramachandran
IEEE Trans. Nanotechnol., 10, 513-519 (2011)
12. Ethanol gas sensing of Mn-doped CoFe 2 O 4 nanoparticles
P. Indra Devi, N. Rajkumar, B. Renganathan, D. Sastikumar, and K. Ramachandran
IEEE Sensors J., 11, 1395-1402 (2011)
11. On the possibility of ferromagnetism in CdO:Mn at room temperature
N. Rajkumar, V.M. Susila, and K. Ramachandran
J. Exp. Nanosci., 6, 389-398 (2011)
10. Dumbbell shaped ZnO nanorods: Growth and Ch