Satyaprakash Sahoo

Surface energy engineering for tunable wettability through controlled synthesis of MoS2.

MoS2 is an important member of the transition metal dichalcogenides that is emerging as a potential 2D atomically thin layered material for low power electronic and optoelectronic applications. However, for MoS2 a critical fundamental question of significant importance is how the surface energy and hence the wettability is altered at the nanoscale in particular, the role of crystallinity and orientation. This work reports on the synthesis of large area MoS2 thin films on insulating substrates (SiO2/Si and Al2O3) with different surface morphology via vapor phase deposition by varying the growth temperatures. The samples were examined using transmission electron microscopy and Raman spectroscopy. From contact angle measurements, it is possible to correlate the wettability with crystallinity at the nanoscale. The specific surface energy for few layers MoS2 is estimated to be about 46.5 mJ/m(2). Moreover a layer thickness-dependent wettability study suggests that the lower the thickness is, the higher the contact angle will be. Our results shed light on the MoS2-water interaction that is important for the development of devices based on MoS2 coated surfaces for microfluidic applications.

Studies on structural, dielectric, and transport properties of Ni0.65Zn0.35Fe2O4

We report the crystal structure, dielectric, transport, and magnetic properties of Ni0.65Zn0.35Fe2O4. Rietveld refinement results of X-ray diffraction patterns confirm the phase formation of the material with cubic crystal structure ( Fd3¯m). The frequency dependent ac conductivity behavior obeys the Jonschers power law and is explained using the jump relaxation model. The observed behavior of temperature dependent bulk conductivity is attributed to the variable-range hopping of localized polarons. The correlation of polaron conduction and high permittivity behavior of NZFO is established on the basis of long range and short range conduction mechanisms. The complex impedance spectra clearly show the contribution of both grain and grain boundary effect on the electrical properties.

Surface optical Raman modes in InN nanostructures

Raman spectroscopic investigations are carried out on one-dimensional nanostructures of InN, such as nanowires and nanobelts synthesized by chemical vapor deposition. In addition to the optical phonons allowed by symmetry A1, E1, and E2 (high) modes, two additional Raman peaks are observed around 528 and 560 cm−1 for these nanostructures. Calculations for the frequencies of surface optical (SO) phonon modes in InN nanostructures yield values close to those of the new Raman modes. A possible reason for large intensities for SO modes in these nanostructures is also discussed.

Studies of the switchable photovoltaic effect in co-substituted BiFeO3 thin films

We report the photovoltaic properties of doped ferroelectric [Bi0.9La0.1][Fe0.97Ti0.02Zr0.01]O3 (BLFTZO) thin films. Polycrystalline BLFTZO films were fabricated on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition technique. Al-doped ZnO transparent top electrodes complete the ZnO:Al/BLFTZO/Pt metal-ferroelectric-metal capacitor structures. BLFTZO showed switchable photoresponse in both polarities. The open circuit voltage (VOC) and short circuit current (JSC) were found to be ∼0.022 V and ∼650 μA/cm2, respectively after positive poling, whereas significant difference in VOC ∼ 0.018V and JSC ∼ 700 μA/cm2 was observed after negative poling. The observed switchable photocurrent and photovoltage responses are explained on the basis of polarization flipping in BLFTZO due to the applied poling field.

Polarized Raman scattering in monolayer, bilayer, and suspended bilayer graphene

Polarized resonant Raman scattering studies have been performed on monolayer, bilayer, and suspended bilayer graphene. The intensity of the G band is found to be unchanged with polarization in monolayer and bilayer graphene, whereas it shows polarization dependence in suspended bilayer graphene. The 2D mode shows strong polarization depended behavior for all the samples, irrespective of layer thickness. It is found that the intensity of the 2D peak is at a maximum when the incident photon is parallel to the scattered photon and at a minimum when these are perpendicular to each other. This effect has been explained on the basis of anisotropic photon scattering through the nodes at the K-point of the Brillouin zone in graphene during optical absorption.

Phonons and magnetic excitation correlations in weak ferromagnetic YCrO3

Here, we report the temperature dependent Raman spectroscopic studies on orthorhombically distorted perovskite YCrO3 over a temperature range of 20–300 K. Temperature dependence of DC-magnetization measurements under field cooled and zero field cooled protocols confirmed a Neel transition at TN ∼ 142 K. Magnetization isotherms recorded at 125 K show a clear loop opening without any magnetization saturation up to 20 kOe, indicating a coexistence of antiferromagnetic (AFM) and weak ferromagnetic (WFM) phases. Estimation of exchange constants using mean-field approximation further confirm the presence of a complex magnetic phase below TN. Temperature evolution of Raman line-shape parameters of the selected modes (associated with the octahedral rotation and A(Y)-shift in the unit-cell) reveal an anomalous phonon shift near TN. An additional phonon anomaly was identified at T* ∼ 60 K, which could possibly be attributed to the change in the spin dynamics. Moreover, the positive and negative shifts in Raman freq...

Observation of torsional mode in CdS1−xSex nanoparticles in a borosilicate glass

Phonon modes found in low-frequency Raman scattering from CdS1−xSex nanocrystals embedded in a borosilicate glass arising from confined acoustic phonons are investigated. In addition to the breathing and quadrupolar modes, two additional modes are found in the spectra. In order to assign the new modes, confined acoustic phonon frequencies are calculated using the complex frequency model, the core-shell model, and the Lamb model. Based on the ratio of the frequencies of the new modes to those of the quadrupolar mode, the new modes are assigned to the first overtone of the quadrupolar mode (l=2, n=1) and to the l=1, n=0 torsional mode. To confirm the assignment of the torsional mode, the polarized Raman scattering measurements in parallel-polarized (VV) and perpendicular-polarized (VH) symmetries are performed. The torsional mode is present in both VV and VH symmetries. The appearance of the forbidden torsional mode is attributed to the near-spherical shape of the nanoparticle found from high-resolution tra...

Thermal conductivity of freestanding single wall carbon nanotube sheet by Raman spectroscopy.

Thermal properties of single wall carbon nanotube sheets (SWCNT-sheets) are of significant importance in the area of thermal management, as an isolated SWCNT possesses high thermal conductivity of the value about 3000 W m(-1) K(-1). Here we report an indirect method of estimating the thermal conductivity of a nanometer thick suspended SWCNT-sheet by employing the Raman scattering technique. Tube diameter size is examined by the transmissions electron microscopy study. The Raman analysis of the radial breathing modes predicts narrow diameter size distribution with achiral (armchair) symmetry of the constituent SWCNTs. From the first order temperature coefficient of the A1g mode of the G band along with the laser power dependent frequency shifting of this mode, the thermal conductivity of the suspended SWCNT-sheet is estimated to be about ∼18.3 W m(-1) K(-1). Our theoretical study shows that the thermal conductivity of the SWCNT-sheet has contributions simultaneously from the intratube and intertube thermal transport. The intertube thermal conductivity (with contributions from the van der Waals interaction) is merely around 0.7 W m(-1) K(-1), which is three orders smaller than the intratube thermal conductivity, leading to an abrupt decrease in the thermal conductivity of the SWCNT-sheet as compared to the reported value for isolated SWCNT.

Raman scattering from surface optical phonon in diameter modulated AlN nanotips

We investigate the optical phonons in AlN nanotips using Raman spectroscopy. Apart from the group theoretically allowed optical phonons, an additional phonon mode at 850 cm−1 has been observed. Furthermore, the peak position has shown a significant redshift and its intensity increases dramatically with the change in dielectric medium. In view of its strong response to change in dielectric medium and good agreement with calculated surface optical phonon frequency in AlN, the observed phonon mode is assigned as surface optical phonon, which could be attributed to the diameter modulation in case of AlN nanotip.

Temperature dependent Raman scattering studies of three dimensional topological insulators Bi2Se3

We investigate the temperature dependent (83 K≤T≤523 K) frequency shift of 2Ag1 and 1Eg2 phonon modes in the three dimensional topological insulator Bi2Se3, using Raman spectroscopy. The high quality single crystals of Bi2Se3 were grown using a modified Bridgman technique and characterized by Laue diffraction and high resolution transmission electron microscopy. A significant broadening in the line shape and red-shift in the frequencies were observed with increase in temperature. Polarized Raman scattering measurement shows a strong polarization effect of Ag1 and Ag2 phonon modes which confirms the good quality single crystals of Bi2Se3. Temperature co-efficient for A1g1, Eg2, and A1g2 modes was estimated to be −1.44 × 10−2, −1.94 × 10−2, and −1.95 × 10−2 cm−1∕K, respectively. Our results shed light on anharmonic properties of Bi2Se3.