Browsing by Author "C.K. Mahadevan"

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Investigation on the synthesis and quantum confinement effects of pure and Mn2+ added Zn(1-x)CdxS nanocrystals
(2011) R. Sakthi Sudar Saravanan; D. Pukazhselvan; C.K. Mahadevan
Zn(1-x)CdxS and Zn(1-x)Cd xS:Mn2+ semiconductor quantum dots (2-4 nm) have been prepared by a novel solvothermal route assisted microwave heating method. The growth parameters governing the smaller size and higher yield have been optimized. The synthesized QDs exhibit a significant blue shift as compared to their corresponding bulk counterpart in the UV-vis optical absorption spectrum. The dielectric constant value varies from 2.79 to 6.17 (at 40 °C, 1 kHz) depending upon the composition of the alloy; lower value corresponds to Zn 0.75Cd0.25S:Mn2+ and the higher value corresponds to Zn0.25Cd0.75S:Mn2+. The crystallite size to exciton bohr radius ratio being <1 indicates a strong quantum confinement effect in both CdS and ZnS QDs. The quantum confinement effect exists in the sequence of ZnS:Mn2+ < Zn (1-x)CdxS:Mn2+ (x < 0.5) < ZnS < Zn(1-x)CdxS < CdS < CdS:Mn2+. © 2011 Elsevier B.V. All rights reserved.
Studies on the synthesis and characterization of Zn1-xCd xS and Zn1-xCdxS:Mn2+ semiconductor quantum dots
(2011) R. Sakthi Sudar Saravanan; D. Pukazhselvan; C.K. Mahadevan
Quantum dots (3-4 nm) of Zn1-xCdxS (both free of Mn2+ and with Mn2+ incorporated) were synthesized through a novel solvothermal-microwave irradiation technique. Detailed structural analysis of the Zn1-xCdxS and Zn1-xCd xS:Mn2+ (x = 0, 0.25, 0.5, 0.75 and 1) materials was carried out using powder X-ray diffraction technique. For all the compositions, the crystallite size was controlled to less than 1.5 nm. The optical energy gap for Zn1-xCdxS was found to vary from 3.878 to 2.519 eV and for Zn1-xCd1-xS:Mn2+ it varies from 3.830 to 2.442 eV when x is increased from 0 to 1. Overall, the optical energy gap could be tuned from a minimum of 2.442 eV to a maximum of 3.878 eV. DC conductivity analysis (from 40C to 150°C) and electrical energy gap analysis for all the compositions were also performed. The dc conductivity for Zn 1-xCd1-xS solid solutions varies from 0.3840 × 10-10 to 8.7782 × 10-10 mho/m at 150°C and for Zn1-xCd1-xS:Mn2+ it varies from 0.5751 × 10-10 to 9.8078 × 10-10 mho /m at 150°C (for x = 0 to x = 1). The method of synthesis and the results observed in this investigation may assist in the fabrication of optical devices when the required operational performance falls under the range observed in the study. © 2010 2011 Taylor & Francis.
Studies on the synthesis of cubic ZnS quantum dots, capping and optical-electrical characteristics
(2012) R. Sakthi Sudar Saravanan; D. Pukazhselvan; C.K. Mahadevan
This paper presents a comparative analysis of ZnS QDs synthesized by conventional and microwave heating techniques using zinc acetate and sodium sulphide reactants. The size of the quantum dots achieved by the latter technique (∼3 nm) is at least 30 times smaller than the former technique. Incorporation of excess Na 2S and microwave treatment are the important factors responsible for controlling the size of ZnS nanocrystals. Furthermore, the distribution of quantum dots is highly influenced by the addition of small amount of NaOH. The UV-vis analysis reveals that the band gap can be widened up to 3.94 eV (correspond to ∼3 nm ZnS) from 3.67 eV (correspond to bulk ZnS). Surprisingly better conductivity is observed for the widest band gap ZnS of the present study; this could be due to defects/vacancies present in the system and its influence in the band structure. The higher conductivity value is supported by the smaller activation energy value, smaller dielectric constant and higher dielectric loss, etc. The conduction is further explained by thermionic emission mechanism. © 2011 Elsevier B.V.