A researcher in the field of in-situ rare earth doped III-nitrides nanostructures for optoelectronics and spintronic applications. Supervisors: Dr. Ratnakar Palai
The In x Ga 1Àx N thin films with indium content of x = 14-18 at.% were successfully grown by usi... more The In x Ga 1Àx N thin films with indium content of x = 14-18 at.% were successfully grown by using molecular beam epitaxy (MBE) at high growth temperatures from 650°C to 800°C. In situ reflection high-energy electron diffraction (RHEED) of the In x Ga 1Àx N films confirmed the Stranski-Krastanov growth mode. X-ray diffraction (XRD) of the films confirmed their highly crystalline nature having c-axis orientation with a small fraction of secondary InN phase admixture. High-resolution cross-sectional scanning electron microscopy images showed two-dimensional epilayers growth with thickness of about $260 nm. The high growth temperature of In x Ga 1Àx N epilayers is found to be favorable to facilitate more GaN phase than InN phase. All the fundamental electronic states of In, Ga, and N were identified by x-ray photoelectron spectroscopy (XPS) and the indium composition has been calculated from the obtained XPS spectra with CASAXPS software. The composition calculations from XRD, XPS and photoluminescence closely match each other. The biaxial strain has been calculated and found to be increasing with the In content. Growing In x Ga 1Àx N at high temperatures resulted in the reduction in stress/ strain which affects the radiative electron-hole pair recombination. The In x Ga 1Àx N film with lesser strain showed a brighter and stronger green emission than films with the larger built-in strain. A weak S-shaped near band edge emission profile confirms the relatively homogeneous distribution of indium.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Cadmium selenide (CdSe) thin films were deposited on indium tin oxide (ITO) coated glass substrat... more Cadmium selenide (CdSe) thin films were deposited on indium tin oxide (ITO) coated glass substrates using pulsed laser deposition (PLD) technique under different growth temperatures. Samples were investigated for their structural, morphological, and optical properties through X-ray diffraction (XRD), atomic force microscopy (AFM), and UV-Vis-NIR spectroscopy. AFM analysis revealed that the surface roughness of the as-grown CdSe thin films increased with the increase in deposition temperature. The optical constants and film thickness were obtained from spectroscopic ellipsometry analysis and are discussed in detail. The optical band gap of the as-grown CdSe thin films, calculated from the Tauc plot analysis, matched with the ellipsometry measurements, with a band gap of ~1.71 eV for a growth temperature range of 150 °C to 400 °C. The CdSe thin films were found to have a refractive index of ~3.0 and extinction coefficient of ~1.0, making it a suitable candidate for photovoltaics.
We report a detailed study on the dynamical response of localized electron hopping and dipole rel... more We report a detailed study on the dynamical response of localized electron hopping and dipole relaxation in bulk polycrystals of Zn diluted Cuprospinel (Cu1 − x Zn x Fe2O4). The variations in the dielectric dispersion and ac-resistivity (ρ ac ) were analyzed over a wide temperature (77 K ≤T ≤ 823 K) and frequency (20 Hz ≤ f ≤ 20 MHz) window for a critical composition x c = 0.4. The variation of ϵ R (f, T) followed the Maxwell–Wagner type polarization mechanism in-line with the Koops phenomenological theory. Our analysis of ρ ac (T, f) provide strong evidence to the Mott’s variable range hopping of charge transport between the localized states at low temperatures, however, thermally-activated Arrhenius like behaviour was noticed at high temperatures with E A = 656 meV for x c = 0.4. Moreover, electric modulus spectroscopic studies (M *(f, T)) reveals two distinct types of relaxation phenomena: (i) the short-range oscillations of the charge carriers within the potential well of grains and (ii) the long-range movement of charge carriers across the grain boundaries. The depressed semi-circles of the Nyquist plots and lower values of non-exponential parameter extracted from M *(f, T) suggest the non-Debye type relaxation process present in the system with a widespread distribution of relaxation times. The frequency exponent (s(x, T)) study of Jonscher’s power law reveals that the ac-conductivity follows small-polaron tunnelling followed by the correlated-barrier-hopping mechanism for x < 0.1. However, for x ≥ x c reorientational hopping mechanism is predominant, except for T > 400 K, where thermally activated Arrhenius-type conduction of charge carriers is prevalent in this spinel system. Furthermore, the tetragonally (I41/amd) distorted systems (x ≤ 0.05) exhibit less activation energy (E A − VRH ) values as compared to those of cubic-spinel symmetry (Fd-3m) which saturates at 130 meV for 0.1 ≤ x ≤ 0.6. Compositional dependent tunability of the above discussed parameters may open a constructive approach to design low energy-loss and high-resistive electromagnetic elements for microwave devices which is the key significance of the present study.
In this study, we systematically investigated the phase transition (cubic (90°) → rhombohedral (6... more In this study, we systematically investigated the phase transition (cubic (90°) → rhombohedral (60.01°)) and its role in the electronic structure, dielectric, and magnetic behavior of Ni 1−x Na x O (0.02 x 0.2). X-ray photoelectron spectroscopy indicated non-local screening of the divalent Ni by oxygen and monovalent sodium ions together with giant multiplet splitting. At 310 K, the longitudinal optical phonon mode (500 cm −1) dominated over all the vibrational excitations, but for the dilute dispersion of Na, the transverse optical phonon mode (455 cm −1) exhibited the highest intensity among all the modes. Even at a very low level (x∼ 0.02) of Na substitution, the intensity of the 2-magnon mode (1530 cm −1) was significantly suppressed. The onset of the structural transition and antiferromagnetic Néel temperature (T N) shifted to a high temperature region with moderate Na substitution, which has significant implications for the dependence of the relative dielectric permittivity (ε R (T)) and magnetic susceptibility (χ mag (T)) on the temperature. The dependence of the ac-resistivity ρ ac (T, f) on the temperature and frequency followed Mott's variable range-hopping mechanism for charge carriers between the localized states, where the activation energies were highly dependent on the spin (S = 1/2) of the type-II antiferromagnetic system. The magnetization isotherms (M-H) were analyzed using a modified Langevin function = +
We report on photoluminescence (PL) properties of GaN, GaN:Yb, InGaN, and InGaN:Yb thin films gro... more We report on photoluminescence (PL) properties of GaN, GaN:Yb, InGaN, and InGaN:Yb thin films grown on (0001) sapphire substrates by plasma assisted molecular beam epitaxy (MBE). X-ray diffraction pattern of the films confirms c-axis oriented growth. The concentration of Yb and In was obtained by X-ray photoelectron spectroscopy (XPS) and was found to be 5 (± 0.5) at.% and 30 (± 1.5) at.%, respectively. The GaN:Yb and InGaN:Yb thin films show a significant linewidth narrowing in PL spectra compared to GaN and InGaN thin films. This could be attributed to the reduction of the defect related non-radiative recombination paths and suppression of the structural defects and dislocations because of the in situ rare earth (Yb)-doping during the growth. The temperature dependent photoluminescence of GaN:Yb thin film follows the Varshni model, whereas InGaN:Yb film shows a complex S-shaped like behavior, which can be explained by the localization effect using Band-Tail model.
We report a detailed single-crystal and powder neutron diffraction study of Co 2 TiO 4 and Co 2 S... more We report a detailed single-crystal and powder neutron diffraction study of Co 2 TiO 4 and Co 2 SnO 4 between the temperature 1.6 and 80 K to probe the spin structure in the ground state. For both compounds the strongest magnetic intensity was observed for the (111) M reflection due to ferrimagnetic ordering, which sets in below T N = 48.6 and 41 K for Co 2 TiO 4 and Co 2 SnO 4 , respectively. An additional low intensity magnetic reflection (200) M was noticed in Co 2 TiO 4 due to the presence of an additional weak antiferromagnetic component. Interestingly, from both the powder and single-crystal neutron data of Co 2 TiO 4 , we noticed a significant broadening of the magnetic (111) M reflection, which possibly results from the disordered character of the Ti and Co atoms on the B site. Practically, the same peak broadening was found for the neutron powder data of Co 2 SnO 4. On the other hand, from our single-crystal neutron diffraction data of Co 2 TiO 4 , we found a spontaneous increase of particular nuclear Bragg reflections below the magnetic ordering temperature. Our data analysis showed that this unusual effect can be ascribed to the presence of anisotropic extinction, which is associated to a change of the mosaicity of the crystal. In this case, it can be expected that competing Jahn-Teller effects acting along different crystallographic axes can induce anisotropic local strain. In fact, for both ions Ti 3+ and Co 3+ , the 2t g levels split into a lower d xy level yielding a higher twofold degenerate d xz /d yz level. As a consequence, one can expect a tetragonal distortion in Co 2 TiO 4 with c/a < 1, which we could not significantly detect in the present work.
In this article, we report a comparative analysis of various spectroscopic studies including low‐... more In this article, we report a comparative analysis of various spectroscopic studies including low‐temperature ( 25K≤T≤300K ) Raman spectroscopy of cobalt‐orthotitanate ( Co2TiO4 ) and tricobalt‐tetraoxide ( Co3O4 ), and their solid solutions (1−x) Co3O4+x of Co2TiO4 ( 0≤x≤1 (100 wt.%)). For all the lower and intermediate compositions, five Raman‐active modes were recognized at 689, 618, 518, 480 , and 195cm−1 that are associated with A1g , Eg , and 3F2g phonon symmetries. Conversely, pure Co2TiO4 exhibits a broad spectrum of width ∼93.3cm−1 without any signatures of F2g (3) mode. At low‐temperatures (down to 25 K) the A1g and F2g peaks of both Co2TiO4 and Co3O4 shift toward the high‐frequency side with anomalies across the ferrimagnetic Néel temperature ( TN ∼48±5 K) and antiferromagnetic Néel temperature ( TN∼ 30 ± 10 K), respectively. All the investigated samples exhibit two distinct bands at 576 cm−1 ( B1 ) and 665 cm−1 ( B2 ) in the Fourier transform infrared spectra recorded at 300±10 K, associated with the vibrational stretching of the metal–oxygen bonds of length ∼ 195.8 pm (B–O) and ∼ 185.4 pm (A–O), respectively. The intensity of these sharp bands gradually decreases as the crystal structure transforms from normal‐spinel ( a=8.07 Å) to inverse‐spinel structure ( a=8.45 Å). The X‐ray photoelectron spectroscopy (XPS) studies revealed that the Ti was incorporated into the octahedral B‐sites of inverse‐spinel structure of Co2TiO4 . Interestingly, the XPS spectra of Co2TiO4 provide evidence of the trivalent character of Ti instead of tetravalent cationic configuration together with a weak Co3+ character at the octahedral sites. These results are discussed in terms of the binding‐energy (BE) difference between the O‐1s and Ti‐2p 3/2 ( Δ [O–Ti‐2p 3/2 ] = BE(O‐1s) −BE(Ti‐2p 3/2) ) and the mean chemical bond length l[Ti−O] . The peculiarities of all these results in consonance with the crystal‐structure (bond angles and bond lengths) and electron‐spin‐resonance studies are discussed in detail.
We report on photoluminescence (PL) measurements of InGaN:Yb 3+ nanorods (NRs) in combination wit... more We report on photoluminescence (PL) measurements of InGaN:Yb 3+ nanorods (NRs) in combination with crystal field calculations. The observed near band edge (NBE) shift has been discussed in terms of its three possible factors: Indium content fluctuation, strain relaxation and quantum confinement. The transition lines between Stark energy levels of Yb 3+ ion were detected and analyzed. Ytterbium ions were described as having similar lattice locations in InGaN and GaN NRs after a comparison between their luminescent properties. Consequently, we established the experimental energy levels for Yb 3+ ion in InGaN NRs from the assignment of almost all the emission lines to Yb Ga substitutional site. The corresponding crystal field parameters were determined based on the observed Stark splitting of 4f manifolds. The In and Yb concentrations were calculated from the X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) reveal the single crystalline nature of the nanorods.
We report the growth of high indium content InGaN:Yb nanorods grown on c-plane sapphire (0001) su... more We report the growth of high indium content InGaN:Yb nanorods grown on c-plane sapphire (0001) substrates using plasma assisted molecular beam epitaxy. The in situ reflection high energy electron diffraction patterns recorded during and after the growth revealed crystalline nature of the nanorods. The nanorods were examined using electron microscopy and atomic force microscopy. The photoluminescence studies of the nanorods showed the visible emissions. The In composition was calculated from x-ray diffraction, x-ray photoelectron spectroscopy, and the photoluminescence spectroscopy. The In-concentration was obtained from photoluminescence using modified Vegard's law and found to be around 37% for InGaN and 38% for Yb (5 ± 1%)-doped InGaN with a bowing parameter b = 1.01 eV. The Yb-doped InGaN showed significant enhancement in photoluminescence properties compared to the undoped InGaN. The Yb-doped InGaN nanorods demonstrated the shifting of the photoluminescence band at room temp...
the engineering of materials with controlled magnetic properties by means other than a magnetic f... more the engineering of materials with controlled magnetic properties by means other than a magnetic field is of great interest in nanotechnology. In this study, we report engineered magnetic graphene oxide (MGO) in the nanocomposite form of iron oxide nanoparticles (IO)-graphene oxide (GO) with tunable core magnetism and magnetic resonance transverse relaxivity (r 2). These tunable properties are obtained by varying the IO content on GO. The MGO series exhibits r 2 values analogous to those observed in conventional single core and cluster forms of IO in different size regimes-motional averaging regime (MAR), static dephasing regime (SDR), and echo-limiting regime (ELR) or slow motion regime (SMR). The maximum r 2 of 162 ± 5.703 mM −1 s −1 is attained for MGO with 28 weight percent (wt%) content of IO on GO and hydrodynamic diameter of 414 nm, which is associated with the SDR. These findings demonstrate the clear potential of magnetic graphene oxide for magnetic resonance imaging (MRI) applications. Magnetic materials such as superparamagnetic iron oxide nanoparticles (IO) have attracted mounting interest for a wide range of applications in nanomedicine 1-4 , magneto-mechanical actuation 5 , energy storage 6-8 , optoelectronics 9,10 , and environmental remediation 11-15 due to their biocompatibility, hydrophilicity, distinct morphology, and unique magnetic and electric properties. In medical diagnostics, the MRI applications of such materials are of paramount interest, and they are widely used as MRI negative contrast agents (CAs) 16,17 due to their hallmark characteristics of spin-spin or transverse relaxation enhancement. In pursuit of high-performance MRI CAs, the surface modification of IO is mostly executed in the form of core-shell 18,19 and Janus structure 20 using polymer stabilizers along with their controlled shape and size. In addition, with the advances in the research of graphene-based materials, GO has been utilized for surface modification of IO owing to its oxygenated functionalities, i.e., epoxide, hydroxyl, carbonyl, and carboxyl moieties 21,22 and biocompatibility 23,24. These functionalities can serve as the conjugation sites for IO to form GO-based nanocomposites, and in particular, magnetic graphene oxide (MGO). Recently, IO/GO-based nanocomposites have been proposed as T 1 25 and T 2 CAs 4,26,27 for MRI. However, no systematic studies on the tunable magnetic behavior and magnetic resonance (MR) relaxivity of such materials have been reported, and its corresponding size regime correlation remains unsettled. Some groups have recently explored the tunability of magnetic resonance transverse relaxivity (r 2) in single core or cluster forms of IO in the PEGylated core-shell nanostructures. The distinct r 2 values of IO with size range ~5-14 nm were reported via optimization of the coating thickness using PEG with molecular weights of 550, 750, 1000, 2000 and 5000 Da 28. Similarly, the PEGylated raspberry-like nanoclusters of superparamagnetic IO nanocrystals with a diameter range of 30 to 200 nm were reported with distinct r 2 values associated with three size
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2013
Al-doped ZnO films were deposited by the atomic layer deposition (ALD) on both glass and sapphire... more Al-doped ZnO films were deposited by the atomic layer deposition (ALD) on both glass and sapphire (0001) substrates. The Al composition of the films was varied by controlling the Zn:Al pulse cycle ratios. The films were characterized by the atomic force microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and optical measurements. The Film resistivity was measured as a function of Zn:Al cycle ratios as well as temperature for films grown at various substrate temperature used for ALD deposition. The resistivity of the ALD grown films decreases significantly, and so as the increase in the carrier concentration as the cycle ratio increases. The systematic measurements of temperature dependence of resistivity of films at various cycle ratios clearly demonstrate the crossover of the metal-semiconductor-insulator phase with the function of temperature as well as the cycle ratios. The average transmission of all films is greater than 85% and the optical absorption increases significantly in the visible region as the cycle ratio increases. The authors observed a remarkable dependence of photoresistance on electrical conductivity for ALD-grown films with varying cycle ratios, which control the Al content in the film. Our results suggest that Al 3þ ions are incorporated as substitutional or interstitial sites of the ZnO matrix. However, an addition of an excessive amount of Al content causes the formation of Al 2 O 3 and related clusters as carrier traps opposed to electron donors, resulting in an increase in the resistivity and other associated phenomena.
Tungsten-doped (W-doped) zinc oxide (ZnO) nanostructures were synthesized on quartz substrates by... more Tungsten-doped (W-doped) zinc oxide (ZnO) nanostructures were synthesized on quartz substrates by pulsed laser and hot filament chemical vapor co-deposition technique under different oxygen pressures and doping levels. We studied in detail the morphological, structural and optical properties of W-doped ZnO by SEM, XPS, Raman scattering, and optical transmission spectra. A close correlation among the oxygen pressure, morphology, W concentrations and the variation of band gaps were investigated. XPS and Raman measurements show that the sample grown under the oxygen pressure of 2.7 Pa has the maximum tungsten concentration and best crystalline structure, which induces the redshift of the optical band gap. The effect of W concentration on the change of morphology and shift of optical band gap was also studied for the samples grown under the fixed oxygen pressure of 2.7 Pa.
Proceedings of Spie the International Society For Optical Engineering, Feb 1, 2010
The combination of sample translation and line focusing by cylindrical optics is shown to be a co... more The combination of sample translation and line focusing by cylindrical optics is shown to be a convenient and highly effective way of generating laser induced coherent periodic surface structures (LIPSS) in TiO2 over significantly extended areas. Compared to known techniques based on a sample translation relative to a circular symmetric focus, the approach is much less time consuming and requires only a single translation stage. The capability of the method to form both high and low spatial frequency LIPSS (HSFL, LSFL) at the second harmonic wavelengths of a Ti:sapphire-laser (around 400 nm) at properly chosen scanning velocity and laser pulse energies is demonstrated. Structured multi-mm2 areas with periods of 80 nm and 325 nm were obtained corresponding to distinct sets of optimized parameters. Furthermore, the appearance of nano-bumps on 30 nm scale on the surface of the LSFL is reported. Basic technical issues are discussed and potential applications of LIPSS in rutile-type TiO2 like superwetting, friction control, catalysis and photovoltaic are proposed.
Synthesis and Photonics of Nanoscale Materials VII, 2010
The combination of sample translation and line focusing by cylindrical optics is shown to be a co... more The combination of sample translation and line focusing by cylindrical optics is shown to be a convenient and highly effective way of generating laser induced coherent periodic surface structures (LIPSS) in TiO 2 over significantly extended areas. Compared to known techniques based on a sample translation relative to a circular symmetric focus, the approach is much less time consuming and requires only a single translation stage. The capability of the method to form both high and low spatial frequency LIPSS (HSFL, LSFL) at the second harmonic wavelengths of a Ti:sapphire-laser (around 400 nm) at properly chosen scanning velocity and laser pulse energies is demonstrated. Structured multi-mm 2 areas with periods of 80 nm and 325 nm were obtained corresponding to distinct sets of optimized parameters. Furthermore, the appearance of nano-bumps on 30 nm scale on the surface of the LSFL is reported. Basic technical issues are discussed and potential applications of LIPSS in rutile-type TiO 2 like superwetting, friction control, catalysis and photovoltaic are proposed.
The In x Ga 1Àx N thin films with indium content of x = 14-18 at.% were successfully grown by usi... more The In x Ga 1Àx N thin films with indium content of x = 14-18 at.% were successfully grown by using molecular beam epitaxy (MBE) at high growth temperatures from 650°C to 800°C. In situ reflection high-energy electron diffraction (RHEED) of the In x Ga 1Àx N films confirmed the Stranski-Krastanov growth mode. X-ray diffraction (XRD) of the films confirmed their highly crystalline nature having c-axis orientation with a small fraction of secondary InN phase admixture. High-resolution cross-sectional scanning electron microscopy images showed two-dimensional epilayers growth with thickness of about $260 nm. The high growth temperature of In x Ga 1Àx N epilayers is found to be favorable to facilitate more GaN phase than InN phase. All the fundamental electronic states of In, Ga, and N were identified by x-ray photoelectron spectroscopy (XPS) and the indium composition has been calculated from the obtained XPS spectra with CASAXPS software. The composition calculations from XRD, XPS and photoluminescence closely match each other. The biaxial strain has been calculated and found to be increasing with the In content. Growing In x Ga 1Àx N at high temperatures resulted in the reduction in stress/ strain which affects the radiative electron-hole pair recombination. The In x Ga 1Àx N film with lesser strain showed a brighter and stronger green emission than films with the larger built-in strain. A weak S-shaped near band edge emission profile confirms the relatively homogeneous distribution of indium.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Cadmium selenide (CdSe) thin films were deposited on indium tin oxide (ITO) coated glass substrat... more Cadmium selenide (CdSe) thin films were deposited on indium tin oxide (ITO) coated glass substrates using pulsed laser deposition (PLD) technique under different growth temperatures. Samples were investigated for their structural, morphological, and optical properties through X-ray diffraction (XRD), atomic force microscopy (AFM), and UV-Vis-NIR spectroscopy. AFM analysis revealed that the surface roughness of the as-grown CdSe thin films increased with the increase in deposition temperature. The optical constants and film thickness were obtained from spectroscopic ellipsometry analysis and are discussed in detail. The optical band gap of the as-grown CdSe thin films, calculated from the Tauc plot analysis, matched with the ellipsometry measurements, with a band gap of ~1.71 eV for a growth temperature range of 150 °C to 400 °C. The CdSe thin films were found to have a refractive index of ~3.0 and extinction coefficient of ~1.0, making it a suitable candidate for photovoltaics.
We report a detailed study on the dynamical response of localized electron hopping and dipole rel... more We report a detailed study on the dynamical response of localized electron hopping and dipole relaxation in bulk polycrystals of Zn diluted Cuprospinel (Cu1 − x Zn x Fe2O4). The variations in the dielectric dispersion and ac-resistivity (ρ ac ) were analyzed over a wide temperature (77 K ≤T ≤ 823 K) and frequency (20 Hz ≤ f ≤ 20 MHz) window for a critical composition x c = 0.4. The variation of ϵ R (f, T) followed the Maxwell–Wagner type polarization mechanism in-line with the Koops phenomenological theory. Our analysis of ρ ac (T, f) provide strong evidence to the Mott’s variable range hopping of charge transport between the localized states at low temperatures, however, thermally-activated Arrhenius like behaviour was noticed at high temperatures with E A = 656 meV for x c = 0.4. Moreover, electric modulus spectroscopic studies (M *(f, T)) reveals two distinct types of relaxation phenomena: (i) the short-range oscillations of the charge carriers within the potential well of grains and (ii) the long-range movement of charge carriers across the grain boundaries. The depressed semi-circles of the Nyquist plots and lower values of non-exponential parameter extracted from M *(f, T) suggest the non-Debye type relaxation process present in the system with a widespread distribution of relaxation times. The frequency exponent (s(x, T)) study of Jonscher’s power law reveals that the ac-conductivity follows small-polaron tunnelling followed by the correlated-barrier-hopping mechanism for x < 0.1. However, for x ≥ x c reorientational hopping mechanism is predominant, except for T > 400 K, where thermally activated Arrhenius-type conduction of charge carriers is prevalent in this spinel system. Furthermore, the tetragonally (I41/amd) distorted systems (x ≤ 0.05) exhibit less activation energy (E A − VRH ) values as compared to those of cubic-spinel symmetry (Fd-3m) which saturates at 130 meV for 0.1 ≤ x ≤ 0.6. Compositional dependent tunability of the above discussed parameters may open a constructive approach to design low energy-loss and high-resistive electromagnetic elements for microwave devices which is the key significance of the present study.
In this study, we systematically investigated the phase transition (cubic (90°) → rhombohedral (6... more In this study, we systematically investigated the phase transition (cubic (90°) → rhombohedral (60.01°)) and its role in the electronic structure, dielectric, and magnetic behavior of Ni 1−x Na x O (0.02 x 0.2). X-ray photoelectron spectroscopy indicated non-local screening of the divalent Ni by oxygen and monovalent sodium ions together with giant multiplet splitting. At 310 K, the longitudinal optical phonon mode (500 cm −1) dominated over all the vibrational excitations, but for the dilute dispersion of Na, the transverse optical phonon mode (455 cm −1) exhibited the highest intensity among all the modes. Even at a very low level (x∼ 0.02) of Na substitution, the intensity of the 2-magnon mode (1530 cm −1) was significantly suppressed. The onset of the structural transition and antiferromagnetic Néel temperature (T N) shifted to a high temperature region with moderate Na substitution, which has significant implications for the dependence of the relative dielectric permittivity (ε R (T)) and magnetic susceptibility (χ mag (T)) on the temperature. The dependence of the ac-resistivity ρ ac (T, f) on the temperature and frequency followed Mott's variable range-hopping mechanism for charge carriers between the localized states, where the activation energies were highly dependent on the spin (S = 1/2) of the type-II antiferromagnetic system. The magnetization isotherms (M-H) were analyzed using a modified Langevin function = +
We report on photoluminescence (PL) properties of GaN, GaN:Yb, InGaN, and InGaN:Yb thin films gro... more We report on photoluminescence (PL) properties of GaN, GaN:Yb, InGaN, and InGaN:Yb thin films grown on (0001) sapphire substrates by plasma assisted molecular beam epitaxy (MBE). X-ray diffraction pattern of the films confirms c-axis oriented growth. The concentration of Yb and In was obtained by X-ray photoelectron spectroscopy (XPS) and was found to be 5 (± 0.5) at.% and 30 (± 1.5) at.%, respectively. The GaN:Yb and InGaN:Yb thin films show a significant linewidth narrowing in PL spectra compared to GaN and InGaN thin films. This could be attributed to the reduction of the defect related non-radiative recombination paths and suppression of the structural defects and dislocations because of the in situ rare earth (Yb)-doping during the growth. The temperature dependent photoluminescence of GaN:Yb thin film follows the Varshni model, whereas InGaN:Yb film shows a complex S-shaped like behavior, which can be explained by the localization effect using Band-Tail model.
We report a detailed single-crystal and powder neutron diffraction study of Co 2 TiO 4 and Co 2 S... more We report a detailed single-crystal and powder neutron diffraction study of Co 2 TiO 4 and Co 2 SnO 4 between the temperature 1.6 and 80 K to probe the spin structure in the ground state. For both compounds the strongest magnetic intensity was observed for the (111) M reflection due to ferrimagnetic ordering, which sets in below T N = 48.6 and 41 K for Co 2 TiO 4 and Co 2 SnO 4 , respectively. An additional low intensity magnetic reflection (200) M was noticed in Co 2 TiO 4 due to the presence of an additional weak antiferromagnetic component. Interestingly, from both the powder and single-crystal neutron data of Co 2 TiO 4 , we noticed a significant broadening of the magnetic (111) M reflection, which possibly results from the disordered character of the Ti and Co atoms on the B site. Practically, the same peak broadening was found for the neutron powder data of Co 2 SnO 4. On the other hand, from our single-crystal neutron diffraction data of Co 2 TiO 4 , we found a spontaneous increase of particular nuclear Bragg reflections below the magnetic ordering temperature. Our data analysis showed that this unusual effect can be ascribed to the presence of anisotropic extinction, which is associated to a change of the mosaicity of the crystal. In this case, it can be expected that competing Jahn-Teller effects acting along different crystallographic axes can induce anisotropic local strain. In fact, for both ions Ti 3+ and Co 3+ , the 2t g levels split into a lower d xy level yielding a higher twofold degenerate d xz /d yz level. As a consequence, one can expect a tetragonal distortion in Co 2 TiO 4 with c/a < 1, which we could not significantly detect in the present work.
In this article, we report a comparative analysis of various spectroscopic studies including low‐... more In this article, we report a comparative analysis of various spectroscopic studies including low‐temperature ( 25K≤T≤300K ) Raman spectroscopy of cobalt‐orthotitanate ( Co2TiO4 ) and tricobalt‐tetraoxide ( Co3O4 ), and their solid solutions (1−x) Co3O4+x of Co2TiO4 ( 0≤x≤1 (100 wt.%)). For all the lower and intermediate compositions, five Raman‐active modes were recognized at 689, 618, 518, 480 , and 195cm−1 that are associated with A1g , Eg , and 3F2g phonon symmetries. Conversely, pure Co2TiO4 exhibits a broad spectrum of width ∼93.3cm−1 without any signatures of F2g (3) mode. At low‐temperatures (down to 25 K) the A1g and F2g peaks of both Co2TiO4 and Co3O4 shift toward the high‐frequency side with anomalies across the ferrimagnetic Néel temperature ( TN ∼48±5 K) and antiferromagnetic Néel temperature ( TN∼ 30 ± 10 K), respectively. All the investigated samples exhibit two distinct bands at 576 cm−1 ( B1 ) and 665 cm−1 ( B2 ) in the Fourier transform infrared spectra recorded at 300±10 K, associated with the vibrational stretching of the metal–oxygen bonds of length ∼ 195.8 pm (B–O) and ∼ 185.4 pm (A–O), respectively. The intensity of these sharp bands gradually decreases as the crystal structure transforms from normal‐spinel ( a=8.07 Å) to inverse‐spinel structure ( a=8.45 Å). The X‐ray photoelectron spectroscopy (XPS) studies revealed that the Ti was incorporated into the octahedral B‐sites of inverse‐spinel structure of Co2TiO4 . Interestingly, the XPS spectra of Co2TiO4 provide evidence of the trivalent character of Ti instead of tetravalent cationic configuration together with a weak Co3+ character at the octahedral sites. These results are discussed in terms of the binding‐energy (BE) difference between the O‐1s and Ti‐2p 3/2 ( Δ [O–Ti‐2p 3/2 ] = BE(O‐1s) −BE(Ti‐2p 3/2) ) and the mean chemical bond length l[Ti−O] . The peculiarities of all these results in consonance with the crystal‐structure (bond angles and bond lengths) and electron‐spin‐resonance studies are discussed in detail.
We report on photoluminescence (PL) measurements of InGaN:Yb 3+ nanorods (NRs) in combination wit... more We report on photoluminescence (PL) measurements of InGaN:Yb 3+ nanorods (NRs) in combination with crystal field calculations. The observed near band edge (NBE) shift has been discussed in terms of its three possible factors: Indium content fluctuation, strain relaxation and quantum confinement. The transition lines between Stark energy levels of Yb 3+ ion were detected and analyzed. Ytterbium ions were described as having similar lattice locations in InGaN and GaN NRs after a comparison between their luminescent properties. Consequently, we established the experimental energy levels for Yb 3+ ion in InGaN NRs from the assignment of almost all the emission lines to Yb Ga substitutional site. The corresponding crystal field parameters were determined based on the observed Stark splitting of 4f manifolds. The In and Yb concentrations were calculated from the X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) reveal the single crystalline nature of the nanorods.
We report the growth of high indium content InGaN:Yb nanorods grown on c-plane sapphire (0001) su... more We report the growth of high indium content InGaN:Yb nanorods grown on c-plane sapphire (0001) substrates using plasma assisted molecular beam epitaxy. The in situ reflection high energy electron diffraction patterns recorded during and after the growth revealed crystalline nature of the nanorods. The nanorods were examined using electron microscopy and atomic force microscopy. The photoluminescence studies of the nanorods showed the visible emissions. The In composition was calculated from x-ray diffraction, x-ray photoelectron spectroscopy, and the photoluminescence spectroscopy. The In-concentration was obtained from photoluminescence using modified Vegard's law and found to be around 37% for InGaN and 38% for Yb (5 ± 1%)-doped InGaN with a bowing parameter b = 1.01 eV. The Yb-doped InGaN showed significant enhancement in photoluminescence properties compared to the undoped InGaN. The Yb-doped InGaN nanorods demonstrated the shifting of the photoluminescence band at room temp...
the engineering of materials with controlled magnetic properties by means other than a magnetic f... more the engineering of materials with controlled magnetic properties by means other than a magnetic field is of great interest in nanotechnology. In this study, we report engineered magnetic graphene oxide (MGO) in the nanocomposite form of iron oxide nanoparticles (IO)-graphene oxide (GO) with tunable core magnetism and magnetic resonance transverse relaxivity (r 2). These tunable properties are obtained by varying the IO content on GO. The MGO series exhibits r 2 values analogous to those observed in conventional single core and cluster forms of IO in different size regimes-motional averaging regime (MAR), static dephasing regime (SDR), and echo-limiting regime (ELR) or slow motion regime (SMR). The maximum r 2 of 162 ± 5.703 mM −1 s −1 is attained for MGO with 28 weight percent (wt%) content of IO on GO and hydrodynamic diameter of 414 nm, which is associated with the SDR. These findings demonstrate the clear potential of magnetic graphene oxide for magnetic resonance imaging (MRI) applications. Magnetic materials such as superparamagnetic iron oxide nanoparticles (IO) have attracted mounting interest for a wide range of applications in nanomedicine 1-4 , magneto-mechanical actuation 5 , energy storage 6-8 , optoelectronics 9,10 , and environmental remediation 11-15 due to their biocompatibility, hydrophilicity, distinct morphology, and unique magnetic and electric properties. In medical diagnostics, the MRI applications of such materials are of paramount interest, and they are widely used as MRI negative contrast agents (CAs) 16,17 due to their hallmark characteristics of spin-spin or transverse relaxation enhancement. In pursuit of high-performance MRI CAs, the surface modification of IO is mostly executed in the form of core-shell 18,19 and Janus structure 20 using polymer stabilizers along with their controlled shape and size. In addition, with the advances in the research of graphene-based materials, GO has been utilized for surface modification of IO owing to its oxygenated functionalities, i.e., epoxide, hydroxyl, carbonyl, and carboxyl moieties 21,22 and biocompatibility 23,24. These functionalities can serve as the conjugation sites for IO to form GO-based nanocomposites, and in particular, magnetic graphene oxide (MGO). Recently, IO/GO-based nanocomposites have been proposed as T 1 25 and T 2 CAs 4,26,27 for MRI. However, no systematic studies on the tunable magnetic behavior and magnetic resonance (MR) relaxivity of such materials have been reported, and its corresponding size regime correlation remains unsettled. Some groups have recently explored the tunability of magnetic resonance transverse relaxivity (r 2) in single core or cluster forms of IO in the PEGylated core-shell nanostructures. The distinct r 2 values of IO with size range ~5-14 nm were reported via optimization of the coating thickness using PEG with molecular weights of 550, 750, 1000, 2000 and 5000 Da 28. Similarly, the PEGylated raspberry-like nanoclusters of superparamagnetic IO nanocrystals with a diameter range of 30 to 200 nm were reported with distinct r 2 values associated with three size
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2013
Al-doped ZnO films were deposited by the atomic layer deposition (ALD) on both glass and sapphire... more Al-doped ZnO films were deposited by the atomic layer deposition (ALD) on both glass and sapphire (0001) substrates. The Al composition of the films was varied by controlling the Zn:Al pulse cycle ratios. The films were characterized by the atomic force microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and optical measurements. The Film resistivity was measured as a function of Zn:Al cycle ratios as well as temperature for films grown at various substrate temperature used for ALD deposition. The resistivity of the ALD grown films decreases significantly, and so as the increase in the carrier concentration as the cycle ratio increases. The systematic measurements of temperature dependence of resistivity of films at various cycle ratios clearly demonstrate the crossover of the metal-semiconductor-insulator phase with the function of temperature as well as the cycle ratios. The average transmission of all films is greater than 85% and the optical absorption increases significantly in the visible region as the cycle ratio increases. The authors observed a remarkable dependence of photoresistance on electrical conductivity for ALD-grown films with varying cycle ratios, which control the Al content in the film. Our results suggest that Al 3þ ions are incorporated as substitutional or interstitial sites of the ZnO matrix. However, an addition of an excessive amount of Al content causes the formation of Al 2 O 3 and related clusters as carrier traps opposed to electron donors, resulting in an increase in the resistivity and other associated phenomena.
Tungsten-doped (W-doped) zinc oxide (ZnO) nanostructures were synthesized on quartz substrates by... more Tungsten-doped (W-doped) zinc oxide (ZnO) nanostructures were synthesized on quartz substrates by pulsed laser and hot filament chemical vapor co-deposition technique under different oxygen pressures and doping levels. We studied in detail the morphological, structural and optical properties of W-doped ZnO by SEM, XPS, Raman scattering, and optical transmission spectra. A close correlation among the oxygen pressure, morphology, W concentrations and the variation of band gaps were investigated. XPS and Raman measurements show that the sample grown under the oxygen pressure of 2.7 Pa has the maximum tungsten concentration and best crystalline structure, which induces the redshift of the optical band gap. The effect of W concentration on the change of morphology and shift of optical band gap was also studied for the samples grown under the fixed oxygen pressure of 2.7 Pa.
Proceedings of Spie the International Society For Optical Engineering, Feb 1, 2010
The combination of sample translation and line focusing by cylindrical optics is shown to be a co... more The combination of sample translation and line focusing by cylindrical optics is shown to be a convenient and highly effective way of generating laser induced coherent periodic surface structures (LIPSS) in TiO2 over significantly extended areas. Compared to known techniques based on a sample translation relative to a circular symmetric focus, the approach is much less time consuming and requires only a single translation stage. The capability of the method to form both high and low spatial frequency LIPSS (HSFL, LSFL) at the second harmonic wavelengths of a Ti:sapphire-laser (around 400 nm) at properly chosen scanning velocity and laser pulse energies is demonstrated. Structured multi-mm2 areas with periods of 80 nm and 325 nm were obtained corresponding to distinct sets of optimized parameters. Furthermore, the appearance of nano-bumps on 30 nm scale on the surface of the LSFL is reported. Basic technical issues are discussed and potential applications of LIPSS in rutile-type TiO2 like superwetting, friction control, catalysis and photovoltaic are proposed.
Synthesis and Photonics of Nanoscale Materials VII, 2010
The combination of sample translation and line focusing by cylindrical optics is shown to be a co... more The combination of sample translation and line focusing by cylindrical optics is shown to be a convenient and highly effective way of generating laser induced coherent periodic surface structures (LIPSS) in TiO 2 over significantly extended areas. Compared to known techniques based on a sample translation relative to a circular symmetric focus, the approach is much less time consuming and requires only a single translation stage. The capability of the method to form both high and low spatial frequency LIPSS (HSFL, LSFL) at the second harmonic wavelengths of a Ti:sapphire-laser (around 400 nm) at properly chosen scanning velocity and laser pulse energies is demonstrated. Structured multi-mm 2 areas with periods of 80 nm and 325 nm were obtained corresponding to distinct sets of optimized parameters. Furthermore, the appearance of nano-bumps on 30 nm scale on the surface of the LSFL is reported. Basic technical issues are discussed and potential applications of LIPSS in rutile-type TiO 2 like superwetting, friction control, catalysis and photovoltaic are proposed.
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Papers by Kiran Dasari