arXiv subjects

K. Ganesan

Publications and source records attributed to K. Ganesan.

At least 19 recordsLinked to original sources

arXiv2026

Electrochemical doping in H-terminated diamond films: Impact of O-functionalization and insights from in-situ Raman spectro electrochemistry

The p-type surface conductivity of H-terminated diamond (HD, H-diamond) has created new path ways for developing diamond based electronic devices as well as chemical and bio-sensors. However, the hydrophobic nature of the HD surface can negatively impact device performance due to its low wettability. Herein, we report the study on polymer electrolyte-gated field effect transistors (EGFETs) fabricated using pristine and partially O-terminated HD films. The HD surface is transformed from hydrophobic to moderate hydrophilic by partial O-termination. Also, the sheet resistance of the HD surface increases from 7.6 to 18.7 k-Ohms per sq. while the sheet hole density decreases from 10.5 to 4.8 x 10^12 cm^-2 upon partial O-termination. Consequently, the ON - OFF ratio of the EGFET devices decreases from ~ 40 to 14 and the maximum transconductance declines from of -150 to -7.9 micro-seimens per V, but the areal capacitance increases from ~ 7.8 to 27.1 microFarad per cm^2 with partial ozonation on HD surface. In addition the in situ Raman measurements in HD EGFET provide direct experimental evidence of a gating-induced blue shift and linewidth broadening of the diamond Raman band which are associated with strong electron phonon coupling. This work highlights the significant impact of the partial O-termination on the performance of the HD EGFET devices and effect of electrochemical gating on the phonon behaviour of the H-diamond.

cond-mat.mtrl-sci↗

arXiv2025

Role of temperature oscillation in growth of large-grain CdZnTe single crystal by traveling heater method

Self-nucleation in CdZnTe crystal growth remains a significant challenge, despite numerous attempts to achieve large-grain single crystals by restricting multi-nucleation during growth process using the traveling heater method. In this study, we present a novel approach to achieve large-grain CdZnTe single crystals by introducing temperature oscillations above the crystallization temperature during the growth process. This method effectively suppresses secondary nucleation and promotes the preferential selection of a single grain during early stage of growth as well as along the growth axis, by reducing multi-nucleation. By adjusting the amplitude and the number of temperature oscillations, we have successfully grown CdZnTe single crystals with dimensions of 20 mm in diameter and 60 mm in length. The resulting crystals exhibited excellent compositional homogeneity, with a nearly constant resistivity of ~ 10^9 Ohm-cm and Te inclusions smaller than 15 microns along the growth axis. Additionally, the crystal elements were of detector grade achieving an energy resolution of 4.5% for gamma radiation at 662 keV from a 137Cs source in a quasi-hemispherical geometry. This study highlights the critical role of temperature oscillations in controlling secondary nucleation and promoting the formation of large-grain single crystals.

cond-mat.mtrl-sci↗

arXiv2025

Origin of persistent photoconductivity in surface conducting hydrogenated diamond films

The p-type surface conductivity of hydrogen-terminated diamond (HD) has opened up new possibilities for the development of diamond-based electronic devices. However, the origin of the persistent photoconductivity (PPC) observed in surface-conducting HD remains unclear, an understanding that is crucial for advancing HD-based optoelectronic technologies. In this study, we investigate the underlying mechanism of PPC in surface-conducting HD films. A systematic analysis was performed by tuning the carrier density via partial oxygen termination using an ozonation process. With increasing O-termination, both the decay time and the recombination barrier of photoexcited electron-hole pairs were found to decrease significantly, from 232 to 5 seconds, and from ~ 150 to 54 meV, respectively. Temperature-dependent measurements reveal that PPC in HD is influenced by random local potential fluctuations, which delay the recombination of photoexcited carriers. Furthermore, the observed PPC behavior is closely associated with percolative transport processes within the HD film. Importantly, the dependence of PPC on sheet carrier density is correlated with Coulomb interactions between the two-dimensional hole gas and the surface adsorbate layer. This study offers new insights into the PPC mechanism in surface-conducting HD films, contributing to the broader understanding necessary for the design of advanced diamond-based optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Unusual gas sensor response and semiconductor-to-insulator transition in WO3-x nanostructures : The role of oxygen vacancy

WO3-x thinfilms featuring petal-like and lamella-like nanostructures are grown under controlled oxygen partial pressures using hot filament chemical vapor deposition. These synthesized WO3-x nanostructures exhibit monoclinic structure and contain a significant amount of oxygen vacancies (VO) as confirmed by X-ray diffraction and Raman spectroscopy, respectively. These WO3-x nanostructures demonstrate sensor response to both NH3 and NO2 gases even at room temperature. However, the sensor response varies with temperature and analyte gas type. For NH3, the sensors exhibit an increase in resistance behaving like a p-type semiconductor at temperatures below 150 0C while the resistance decreases at higher temperatures, resembling n-type semiconductor behavior. On the other hand, below 150 0C, these sensors display n-type behavior towards NO2 but act like p-type semiconductor at higher temperatures. Further temperature dependent transport studies were performed in these WO3-x nanostructures in the temperature range from 25 to 300 oC, after inducing additional VO in the films through annealing under CH4. The petal-like WO3-x nanostructures display an abrupt change in resistance, indicating insulator-to-semiconductor and semiconductor-to-insulator transitions during heating and cooling cycles respectively, in the temperature range of 100 - 212 0C. In lamella-like WO3-x nanostructures, the resistance is flipped from semiconductor-to-insulator at 300 0C and remains insulating state when cooled down to 30 0C. The abnormal gas sensing behavior and insulator - semiconductor transition is discussed in terms of VO in WO3-x nanostructures.

cond-mat.mtrl-sci↗

arXiv2024

Growth and inelastic light scattering studies on Sr2Nb2O7 single crystals

A crack free and high structural quality Sr2Nb2O7 single crystals were grown by the optical float zone method using optimized growth parameters. Laue pattern confirms single crystalline nature of the grown crystal. Temperature dependent Raman and Brillouin light scattering studies reveal a significant shift in phonon modes across normal to incommensurate phase transition (Tn-in) which occurs ~ 488 K. In the temperature range from 900 down to 500 K, two optical phonon modes about 63 (B1 mode) and 54 cm-1 (A1 mode) were observed. The frequency of A1 mode strongly decreases with an increase in temperature above the Tn-in while the frequency of this mode almost remains constant below the Tn-in. In contrast the frequency of B1 phonon mode is found to increase with temperature in the range of 500 - 900 K but it does not display a significant shift below the phase transition temperature. In addition in the incommensurate phase a new optical phonon mode at 35 cm-1 also begins to appear and exhibits strong stiffening behavior with increase in temperature in the range of 300 - 488 K. Moreover, the anomalous behavior of the acoustic phonon across Tn-in were further probed using Brilliouin scattering. Longitudinal acoustic phonon mode at 41 GHz exhibits strong change in slope near Tn-in. In addition, the transverse acoustic modes at 28.6 and 22.4 GHz also exhibit strong anomalies with minimum in frequency near Tn-in. The inelastic light scattering studies provide valuable information on the phase transition.

cond-mat.mtrl-sci↗

arXiv2024

Structural, optical and mechanical properties of Cr doped \b{eta}-Ga2O3 single crystals

Undoped and Cr doped \b{eta}-Ga2O3 (100) single crystals are grown by optical floating zone method. The full width at half maximum of rocking curve is found to be 106 arcsec for undoped Ga2O3 crystals whereas the 100 and 200 ppm of Cr doped Ga2O3 crystals display multiple rocking curves with large peak widths indicating the presence of structural defects. Raman measurements reveal broadening in the vibrational mode of ~ 350 cm-1 with a shoulder peak indicating the Cr3+ dopants preferentially substitute for Ga3+ at the octahedral sites. Further, the Cr doped Ga2O3 crystals display strong optical absorption bands about 420 and 597 nm in the UV-Vis spectroscopy. Moreover, the observation of sharp characteristic photoluminescence emission lines at 690 and 697 nm also confirms the Cr substitution in the doped crystals. The indentation hardness increases nearly linear from 13.0 to 17.9 GPa whilst the indentation modulus decreases from 224.9 to 202.4 GPa upon Cr doping of 200 ppm in \b{eta}-Ga2O3. The structural defects caused by the Cr doping interrupt the movement of indentation induced dislocations that results in the increase of hardness of the Cr doped \b{eta}-Ga2O3 (100) single crystals.

cond-mat.mtrl-sci↗

arXiv2024

Fabrication of high electron mobility and high photoluminescence quantum yield nanoscrolled monolayer MoS2

We fabricated the 1D nanoscrolled monolayer MoS2 (1L-MoS2) with superior characteristics from 1L-MoS2 film in a facile route, using a suitable organic solvent with optimum surface tension, evaporation rate and dielectric constant, which facilitates the controlled scroll formation. These nanoscrolls behave as multilayers in morphology and monolayer electronically. The nanoscrolls exhibited a direct optical gap with enhanced photoluminescence quantum yield stemming from the weak interlayer coupling among constituent layers and were corroborated by low-frequency Raman measurements and Kelvin probe force microscopy measurements. Furthermore, enhanced photoluminescence emission after annealing uncovers the thermal stability of nanoscrolls. In addition, conducting atomic force microscopy results exhibit a significantly higher photocurrent in the nanoscrolled 1L-MoS2 compared to the 1L-MoS2. We also realized significantly improved field effect transistor device parameters in nanoscrolled 1L-MoS2 devices. In nanoscrolled devices, we report the highest mobility value of 2400 cm2V-1s-1 reported in any form of 1L-MoS2.

cond-mat.mtrl-sci↗

arXiv2023

Growth of large-sized relaxor ferroelectric PZN-PT single crystals by modified flux growth method

A novel bottom-cooling high-temperature solution growth technique is developed for growing large-sized relaxor ferroelectric 0.91Pb(Zn1/3Nb2/3O3)-0.09PbTiO3 (PZN-PT) single crystals. During the growth, an inverse temperature gradient is maintained in the crucible base by flowing air at a controlled rate. This method restricts the number of spontaneously nucleated crystals at crucible bottom, reduces loss of volatile PbO component and favours the growth of large-sized PZN-PT single crystals. Large-sized PZN-PT single crystals of dimensions ~ 22x20x14 mm3 are reproducibly grown by the proposed method. The electrical characteristics of the PZN-PT wafers oriented along the <100>, <010> and <001> directions are investigated. PZN-PT wafers oriented along the <001> direction exhibited superior piezoelectric coefficient (d33) of ~ 2221 pm/V. The homogeneity of the physical parameters is analysed by preparing 10 elements with dimensions of ~5x2.5x2.5 mm3 which were cut from single wafer oriented along the <001> direction. The ferro-, piezo- and dielectric characteristics of these wafers were found to be highly uniform with small standard deviation. The observation of d33 value with less than 2 % deviation from mean value confirms the growth of high quality PZN-PT single crystals.

cond-mat.mtrl-sci↗

arXiv2023

Enhanced sensitivity of partial O-terminated H-diamond for H2S detection at room temperature

The p-type surface conductivity of H-terminated diamond (HD) has opened new path ways to develop diamond based electronic devices, photo-catalysts, chemical and bio-sensors. Herein, we report on the room temperature H2S detection behaviour of pristine HD and the surface modified HD films with partial O-termination (OHD) through ozonation. The response of the pristine HD and partial OHD devices that are ozonated for 30, 60 and 90 s, is found to be ~ 55, 1420, 810 and 95 % respectively, for exposing 900 ppb of H2S under ambient atmosphere at room temperature. Here, the optimally partial OHD sensor displays an enhanced sensitivity by about an order of magnitude due to the catalytic activity of the sparsely populated O-functional groups on HD surface. Moreover, the gas sensor response is found to be higher in wet background atmospheres such as N2 and synthetic air as compared to their respective dry atmospheres. Also, the response curve of these sensors exhibits a peculiar decrease in resistance immediately after exposure to H2S under wet background atmosphere while such oxidative behaviour is absent under dry atmospheres. Based on these observations, the plausible sensing mechanism of these H-diamond based sensors is proposed with the concept of humidity induced H2S hydrolyzation.

cond-mat.mtrl-sci↗

arXiv2022

Studies on tuning surface electronic properties of hydrogenated diamond by oxygen functionalization

Ultra-wide bandgap and the absence of shallow dopants are the major challenges in realizing diamond based electronics. However, the surface functionalization offers an excellent alternative to tune electronic structure of diamonds. Herein, we report on tuning the surface electronic properties of hydrogenated polycrystalline diamond films through oxygen functionalization. The hydrogenated diamond (HD) surface transforms from hydrophobic to hydrophilic nature and the sheet resistance increases from ~ 8 kohms/sq. to over 10 Gohms/sq. with progressive ozonation. The conductive atomic force microscopic (c-AFM) studies reveal preferential higher current conduction on selective grain interiors (GIs) than that of grain boundaries confirming the surface charge transfer doping on these HDs. In addition, the local current conduction is also found to be much higher on (111) planes as compared to (100) planes on pristine and marginally O-terminated HD. However, there is no current flow on the fully O-terminated diamond (OD) surface. Further, X-ray photoelectron spectroscopic (XPS) studies reveal a redshift in binding energy (BE) of C1s on pristine and marginally O-terminated HD surfaces indicating surface band bending whilst the BE shifts to higher energy for OD. Moreover, XPS analysis also corroborate c-AFM study for the possible charge transfer doping mechanism on the diamond films which results in high current conduction on GIs of pristine and partially O-terminated HDs.

cond-mat.mtrl-sci↗

arXiv2022

Raman and photoluminescence spectroscopic studies on structural disorder in oxygen deficient Gd2Ti2O7-d single crystals

We report on Raman and photoluminescence spectroscopic studies on oxygen vacancy induced structural disorder in Gd2Ti2O7-d single crystals grown by optical floating zone technique under argon atmosphere. The oxygen vacancies in Gd2Ti2O7-d wafers decrease with thermal annealing in an air atmosphere. The full width at half maximum of X-ray diffraction rocking curve decreases from 245 to 157 arc-second and the optical transmittance increases from 23 to 87 % (at 1000 nm) upon post growth thermal annealing. Raman spectroscopic studies reveal a monotonic increase in intensity of O-Gd-O (Eg) and Ti-O (A1g) stretching modes with thermal annealing. Since these modes are associated with modulation of oxygen x parameter which is sensitive to Ti-O octahedron distortion, the increase in Raman intensity indicates an improvement in structural ordering of oxygen sub-lattice in Gd2Ti2O7-d. Moreover, the photoluminescence studies also corroborate the Raman analysis in terms of reduction of structural defects associated with oxygen vacancies as a function of thermal annealing. This study demonstrates the effectiveness of using Raman spectroscopy to probe the structural disorder in Gd2Ti2O7-d crystals.

cond-mat.mtrl-sci↗

arXiv2021

Configuration spaces of hard spheres

Hard sphere systems are often used to model simple fluids. The configuration spaces of hard spheres in a three-dimensional torus modulo various symmetry groups are comparatively simple, and could provide valuable information about the nature of phase transitions. Specifically, the topological changes in the configuration space as a function of packing fraction have been conjectured to be related to the onset of first-order phase transitions. The critical configurations for one to twelve spheres are sampled using a Morse-theoretic approach, and are available in an online, interactive database. Explicit triangulations are constructed for the configuration spaces of the two sphere system, and their topological and geometric properties are studied. The critical configurations are found to be associated with geometric changes to the configuration space that connect previously distant regions and reduce the configuration space diameter as measured by the commute time and diffusion distances. The number of such critical configurations around the packing fraction of the solid-liquid phase transition increases exponentially with the number of spheres, suggesting that the onset of the first-order phase transition in the thermodynamic limit is associated with a discontinuity in the configuration space diameter.

cond-mat.stat-mech↗

arXiv2021

Direct microscopic evidence of shear induced graphitization of ultrananocrystalline diamond films

The origin of ultralow friction and high wear resistance in ultrananocrystalline diamond (UNCD) films is still under active debate because of the perplexed tribochemistry at the sliding interface. Herein, we report a comparative study on surface topography and nanoscale friction of tribofilms, in wear tracks of two sets of UNCD films having different structural characteristics. Despite both the films display ultralow coefficient of friction, the UNCD films grown under Ar atmosphere (UNCDAr) exhibit a high wear resistance while the wear rate is higher for the films grown in N2 (UNCDN). Frictional force microscopic (FFM) investigations clearly reveal the manifestation of shear induced graphitization on both the films. However, the wear track of UNCDAr films have a large network of a few layer graphene (FLG) structures over the amorphous carbon tribofilms while only isolated clusters of FLG structures are present in the wear track of UNCDN films. Here, we demonstrate the direct micro-/nanoscopic evidence for the formation of large network of ~ 0.8 - 6 nm thick FLG structures, as a consequence of shear induced graphitization and discuss their decisive role in ultralow friction and wear.

cond-mat.mtrl-sci↗

arXiv2020

Structural, optical and mechanical properties of Y2Ti2O7 single crystal

We report on the growth of Y2Ti2O7 single crystals by optical floating zone technique. X-ray diffraction and Raman spectroscopy studies confirm the structural quality of the crystal. The UV-Vis optical studies reveal that the grown crystals have a high optical transparency with an optical band gap of 3.44 eV. The hardness of Y2Ti2O7 single crystal is measured for the first time using nanoindentation. The measured hardness, indentation and bulk modulus are found to be 16.4$\pm$0.4, 321.1$\pm$6.9 and 243.3$\pm$5.2 GPa respectively, which are higher than its polycrystalline counterpart and its constituent metal oxides, Y2O3 and TiO2.

cond-mat.mtrl-sci↗

arXiv2020

Structural and optical properties of beta irradiated YAlO3 single crystals

We report on the growth, structural and optical properties of YAlO3 single crystals grown by optical floating zone technique. Powder X-ray diffraction and Raman spectroscopic studies confirm the phase purity of the crystals. Raman analysis reveals that the intensity and line-width of Raman bands increase significantly with beta irradiation indicating the formation of structural defects in YAlO3 lattice. The optical properties are studied through UV-visible absorption, and photoluminescence emission and excitation spectroscopies under pre- and post- beta irradiation. The optical studies indicate the presence of Sm and Cr impurities by exhibiting characteristic emission lines in the orange red region. Further, a systematic study on the thermoluminescence (TL) characteristics of the crystal is also carried out at different doses of beta irradiation. The crystals exhibit a prominent TL glow peak at 239 C for less than 5 Gy doses while a weak second glow peak evolves at higher doses. Also, the crystals show a nearly linear dose response in the studied range from 0.1 to 10 Gy. The glow curve analysis reveals that the TL emission obeys the first order kinetics model. Based on the optical studies, the plausible mechanism for the TL glow curve is discussed in terms of the intrinsic defects and impurities that are present in the crystal.

physics.app-ph↗

arXiv2020

Structural, Raman and photoluminescence studies on nanocrystalline diamond films: Effects of ammonia in feedstock

Herein, we report on the improvement of structural quality and enhancement of photoluminescence (PL) emission for an optimally N doped nanocrystalline diamond (NCD) film. Pure and N doped nanocrystalline diamond films are synthesized on Si by hot filament chemical vapour deposition using NH3:CH4:H2 at different nominal N/C ratios viz. 0, 0.13, 0.35, 0.50 and 0.75 in feedstock. X ray diffraction analysis reveal a systematic initial increase and then a decrease in crystallite size with N/C ratio in feedstock. Further, a monotonic increase in Raman line width and peak position of diamond band indicates that the compressive strain in diamond lattice increases as a function of N/C ratio upto 0.50. However, at higher N/C ratio of 0.75, the compressive strain gets relaxed a little and produces a lower strain. Furthermore, a unique Raman mode at 1195 cm-1 is observed corresponding to the C=N-H vibrations indicating a significant N concentration in the NCD films. In addition, visible and UV PL studies reveal the presence of several N related color centres with multiple emission lines in the range of 380 to 700 nm. An optimally N doped diamond film grown at N/C ratio of 0.35 in feedstock shows a significant enhancement in room temperature PL emission at about 505 and 700 nm. This PL enhancement is attributed to H3 and other aggregates of N related defect centres, under 355 and 532 nm laser excitations respectively.

cond-mat.mtrl-sci↗

arXiv2020

Anisotropic three-dimensional weak localization in ultrananocrystalline diamond films with nitrogen inclusions

We present a study of the structural and electronic properties of ultra-nanocrystalline diamond films that were modified by adding nitrogen to the gas mixture during chemical vapour deposition growth. Hall bar devices were fabricated from the resulting films to investigate their electrical conduction as a function of both temperature and magnetic field. Through low-temperature magnetoresistance measurements, we present strong evidence that the dominant conduction mechanism in these films can be explained by a combination of 3D weak localization (3DWL) and thermally activated hopping at higher temperatures. An anisotropic 3DWL model is then applied to extract the phase-coherence time as function of temperature, which shows evidence of a power law dependence in good agreement with theory.

cond-mat.mes-hall↗

arXiv2019

Deep Learning Based Automatic Video Annotation Tool for Self-Driving Car

In a self-driving car, objection detection, object classification, lane detection and object tracking are considered to be the crucial modules. In recent times, using the real time video one wants to narrate the scene captured by the camera fitted in our vehicle. To effectively implement this task, deep learning techniques and automatic video annotation tools are widely used. In the present paper, we compare the various techniques that are available for each module and choose the best algorithm among them by using appropriate metrics. For object detection, YOLO and Retinanet-50 are considered and the best one is chosen based on mean Average Precision (mAP). For object classification, we consider VGG-19 and Resnet-50 and select the best algorithm based on low error rate and good accuracy. For lane detection, Udacity's 'Finding Lane Line' and deep learning based LaneNet algorithms are compared and the best one that can accurately identify the given lane is chosen for implementation. As far as object tracking is concerned, we compare Udacity's 'Object Detection and Tracking' algorithm and deep learning based Deep Sort algorithm. Based on the accuracy of tracking the same object in many frames and predicting the movement of objects, the best algorithm is chosen. Our automatic video annotation tool is found to be 83% accurate when compared with a human annotator. We considered a video with 530 frames each of resolution 1035 x 1800 pixels. At an average each frame had about 15 objects. Our annotation tool consumed 43 minutes in a CPU based system and 2.58 minutes in a mid-level GPU based system to process all four modules. But the same video took nearly 3060 minutes for one human annotator to narrate the scene in the given video. Thus we claim that our proposed automatic video annotation tool is reasonably fast (about 1200 times in a GPU system) and accurate.

cs.CV↗