Kiran Sonawane - Academia.edu (original) (raw)

Papers by Kiran Sonawane

Journal of Luminescence, 2017

In the present work, photoluminescence and photoluminescence excitation (PLE) spectroscopy have b... more In the present work, photoluminescence and photoluminescence excitation (PLE) spectroscopy have been employed to study sp-d exchange interaction of Mn-doped ZnSe nanocrystals (NCs) from room temperature through 10 K. Mn-doped ZnSe NCs of size about 5 nm with zinc blend crystal structure are grown by the nucleation doping strategy. The doped-NCs reveal strong Mn-related emission; surprisingly that shows red shift with the decrease in temperature. At 10 K, four different excitation features related to d-d transitions of Mn 2+ were observed. Crystal field parameter (Dq) and Racah parameters (B and C) are calculated using low temperature PLE spectra recorded by fixing the Mn 2+ related orange luminescence emission. Increased covalency of the metal-ligand bond and the enhanced exchange interaction between different d-electrons is found. Increment in covalency and exchange interaction is corroborated by the appearance of broad feature in electron spin resonance spectroscopy.

Journal of Physics D: Applied Physics, 2017

Implications of grading on emission line width of core-shell nanocrystals Temperature dependent p... more Implications of grading on emission line width of core-shell nanocrystals Temperature dependent photoluminescence (PL) studies were carried out on Cd x Zn 1x S/ZnS alloy core graded shell nanocrystals with varying composition. The effect of grading on the emission line width and hence exciton-phonon coupling is studied. Contrary to the consensus that narrow emission line width is observable with reduction in size (due to increased exciton-acoustic phonon coupling coefficient ), an increased value is noted with size. Based on a theoretical report on graded core-shell NCs, the relationship between electron hole wave function overlap and exciton lifetime was invoked to understand this anomaly. Smaller sized alloy core-shell NCs (CdZnS/ZnS-I) have larger lifetime than that of larger NCs (CdZnS/ZnS-II) as determined from timeresolved spectroscopy. Thus, CdZnS/ZnS-I NCs should have smaller electron hole wave function overlap giving larger effective size of NCs even though the actual size, estimated from TEM, was smaller. Experimental evidence of graded core-shell nanocrystals reveal an additional functionality to control the emission linewidth in NCs.

Journal of Alloys and Compounds, 2017

In spite of the extensive investigations on Co-doped ZnO, existence as well as origin of ferromag... more In spite of the extensive investigations on Co-doped ZnO, existence as well as origin of ferromagnetism remains controversial. Role of charge carriers and defects on the ferromagnetic ordering of Al, Co-codoped ZnO nanocrystals (NCs) having size as small as 5.5 nm is evaluated on the basis of optical spectroscopy, electron spin resonance and Raman spectroscopy. Al, Cocodoped ZnO NCs reveal improved saturation magnetization in a subtle way. Concomitant increase in the charge carriers is vivid from the optical absorption spectra of colloidal codoped NCs. The doping level is maintained low enough so that NCs are free of any secondary or impurity phase. Shape of the M(T) curves is analyzed to elucidate exchange interaction in doped ZnO NCs. The present experimental work indicates that the multiple mechanisms are responsible for stabilizing the ferromagnetic ordering in Co-doped ZnO NCs; thereby controversial reports are published in the literature.

The Journal of Physical Chemistry C, 2016

Graded interface core−shell nanocrystals (NCs) are being investigated to attain highly luminescen... more Graded interface core−shell nanocrystals (NCs) are being investigated to attain highly luminescent systems and are also anticipated to be "blinking-free" at the single-particle level. In the present work, CdSe/ZnSe-graded core−shell NCs with varying confinement potential profile are studied at the single-particle level. The internal structure of NCs is determined with the aid of optical, structural, and chemical probes. Notably, the radiative lifetime for different nanostructures decays monoexponentially. The variation in the radiative lifetime due to differing internal structure is understood on the basis of recently reported first-principles study on different interfaces of core−shell NCs, in particular attributed to varying overlap of electron−hole wave functions. The single NC measurements reveal that the percentage of nonblinking NCs is higher for slowly varying confinement potential. Statistically suppressed blinking (∼80%) and a single exponential PL decay curve, accompanied by a very narrow (∼30 meV) emission line at the single NC level, are observed in graded CdSe/ZnSe core−shell NCs.

The Journal of Physical Chemistry C, 2015

Strain-induced hierarchy of energy levels in CdS/ZnS nanocrystals Aside of size and shape, the st... more Strain-induced hierarchy of energy levels in CdS/ZnS nanocrystals Aside of size and shape, the strain induced by the mismatch of lattice parameters between core and shell in the nanocrystalline regime is an additional degree of freedom to engineer the electron energy levels. Herein, CdS/ZnS core/shell nanocrystals (NCs) with shell thickness up to four monolayers are studied. As a manifestation of strain, the low temperature radiative lifetime measurements indicate a reduction in Stokes shift from 36 meV for CdS to 5 meV for CdS/ZnS with four monolayers of overcoating. Concomitant cross-over of Sand P-symmetric hole levels is observed which can be understood in the framework of theoretical calculations predicting flipping the hierarchy of ground hole state by the strain in CdS NCs. Furthermore, a non-monotonic variation of higher energy levels in strained CdS NCs is discussed.

J. Mater. Chem. C, 2014

Variation in composition provides an additional degree of freedom in nanocrystals design.

Journal of Experimental Nanoscience, 2014

ABSTRACT White light generation is achieved by single-step co-doping of copper and manganese into... more ABSTRACT White light generation is achieved by single-step co-doping of copper and manganese into the robust ZnSe quantum dots (QDs) which were synthesised using a wet chemical route. Photoluminescence (PL) emission spectra revealed three peaks related to blue (ZnSe), green (copper related) and orange (manganese related). The PL spectra indicated no surface and/or trap state related emission. Photoluminescence excitation (PLE) measurements confirmed co-doping of copper and manganese in the same QD. PLE spectra recorded with emission wavelength fixed at copper and manganese showed a band edge at the same position, indicating the incorporation of both copper and manganese in the same QD. Time-resolved PL measurements suggest an atomic like nature of Mn and Cu in ZnSe QDs.

Physica Scripta, 2014

ABSTRACT This paper reports a new method to synthesize Cu-doped ZnSe quantum dots (QDs). Emission... more ABSTRACT This paper reports a new method to synthesize Cu-doped ZnSe quantum dots (QDs). Emission properties are tuned from the blue to the green region simply by increasing the size of the QDs. A red shift in optical absorption of Cu:ZnSe QDs compared with undoped ZnSe QDs is observed. The increase in size of QDs is explained by a change in reaction kinematics. PL measurements revealed both a band edge as well as a copper-related emission. Delocalization of electronic wave functions leads to a shift in the copper-related emission with in size. PL excitation spectra recorded at Cu emission shows ZnSe energy levels along with a feature between 350–370 nm. This feature is assigned to excited energy levels of Cu ions. Variation in electron energy levels as a function of size and on Cu incorporation is mapped.

Springer Proceedings in Physics, 2013

ABSTRACT It is hypothesized that Te forms an isoelectronic trap in ZnSe. These isoelectronic cent... more ABSTRACT It is hypothesized that Te forms an isoelectronic trap in ZnSe. These isoelectronic centers show blue and green band luminescence at low temperature. Quantum confinement effects reveal isoelectronic trap related luminescence at room temperature in contrast to bulk ZnSe1 − yTey. To find the effect of these isoelectronic center on Mn2+ d–d transition luminescence, Mn doped ZnSe0.99Te0.01 QDs are synthesized. Mn doped ZnSe shows dominating orange emission related to Mn2+ d–d transitions. This Mn emission increases at the cost of band edge emission. Addition of Te as small as 1 % in ZnSe strongly quenches photoluminescence of Mn-doped ZnSe QDs showing predominance of Te-isoelectronic centers. Orange emission and near band edge luminescence in Mn doped ZnSe0.99Te0.01 are not correlated as they are in case of Mn-doped ZnSe QDs. Time resolved photoluminescence and photoluminescence excitation study revealed these isoelectronic center changes the recombination path ways. The changes in relaxation path ways are responsible for distinct emission behavior of ZnSe0.99Te0.01QDs.

Solid State Communications, 2014

CdS nanocrystals (NCs) with size 4.3 nm, were coated with ZnS to form CdS/ZnS core shell in order... more CdS nanocrystals (NCs) with size 4.3 nm, were coated with ZnS to form CdS/ZnS core shell in order to examine the effect of lattice strain due to lattice mismatch of 7%. Besides structural analysis, the optical absorption spectrum red-shifts with increasing ZnS shell thickness affirming core-shell nanostructure formation. Radiative life time of CdS NCs remains invariant up to two monolayers of ZnS on CdS NCs and reduces to half the value at three monolayers onwards. The experimental finding is indicative of strain induced change in symmetry of hole wave-function in ground state.

Journal of Luminescence, 2013

Phys.Rev. B (2012). 73. One pot synthesis, growth mechanism and optical properties of Zn 1-x Cd x... more Phys.Rev. B (2012). 73. One pot synthesis, growth mechanism and optical properties of Zn 1-x Cd x Se graded core/shell and alloy nanocrystals

Liquid Crystals, 2018

Supplemental data Figure S1. Optical micrographs showing the appearance of helical lines in KCFLC... more Supplemental data Figure S1. Optical micrographs showing the appearance of helical lines in KCFLC 7S (pure FLC) and CZ QDs doped FLC (composites) at room temperature.