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We demonstrate a non-monotonic evolution of in-plane thermal conductivity of large-area hexagonal boron nitride films with thickness. Wrinkles and defects/impurities are present in these films. Raman spectroscopy, an optothermal non-contact technique, is employed to probe the temperature and laser power dependence property of the Raman active E2ghigh phonon mode, which, in turn, is used to estimate the rise in the temperature of the films under different laser powers. As the conventional Fourier law of heat diffusion cannot be directly employed analytically to evaluate the thermal conductivity of these films with defects and wrinkles, finite-element modeling is used instead. In the model, average heat resistance is used to incorporate an overall near-surface defect structure, and Voronoi cells with contact resistance at the cell boundaries are constructed to mimic the wrinkled domains. The effective in-plane thermal conductivity is estimated to be 87, 55, and 117 W/m K for the 2, 10, and 30 nm-thick films, respectively. We also present a quantitative estimation of the thermal resistance by defects and wrinkles individually to the heat flow. Our study reveals that the defects/impurities render a much higher resistance to heat transfer in the films than wrinkles. [ABSTRACT FROM AUTHOR]

Recently, the electron mobility in wedge-shaped c-GaN nanowall networks has been estimated to cross the theoretical mobility limit for bulk GaN. Significant blue-shift of the bandgap has also been observed. Both the findings are explained in terms of two-dimensional electron gas (2DEG) formed at the central vertical plane of the walls due to the polarization charges at the two inclined faces. Carrier concentration and mobility have earlier been determined from thermoelectric power and conductivity measurements with the help of a statistical model. Due to the network nature of the system, direct measurements of these quantities from Hall experiments are not possible. Search for a better way to estimate mobility in this system thus becomes important. Since, strain can also lead to the blue-shift of the bandgap, it is also imperative to evaluate carefully the role of strain. Here, using Raman spectroscopy, we have estimated carrier concentration and mobility in these nanowall networks with varied average tip-widths. Depth distribution of strain and luminescence characteristics are also studied. The study reveals that strain has no role in the bandgap enhancement. Moreover, the electron mobility, which is determined from the lineshape analysis of the A1(LO)-plasmon coupled mode in Raman spectra, has been found to be significantly higher than the theoretical limit of mobility for bulk GaN for the same electron concentration. These results thus corroborate the picture of polarization induced vertical 2DEG formation in these walls as predicted theoretically. [ABSTRACT FROM AUTHOR]

Wafer-scale thin films of hexagonal boron nitride have exceptional thermal and mechanical properties, which harness the potential use of these materials in two-dimensional electronic, device applications. Along with unavoidable defects, grains, and wrinkles, which develop during the growth process, underlying substrates influence the physical and mechanical properties of these films. Understanding the interactions of these large-scale films with different substrates is, thus, important for the implementation of this 2D system in device fabrication. MOVPE-grown 2 and 30 nm hBN/sapphire films of size 2 in. diameter are delaminated chemically and transferred on quartz, SiO2/Si, and sapphire substrates. The structural characteristics of these films are investigated by employing Raman spectroscopy. Our results suggest that not only the roughness but also the height modulation at the surface of the substrates play a pivotal role in determining substrate-mediated mechanical strain inhomogeneity in these films. The statistical analysis of the spectral parameters provides us with the overall characteristics of the films. Furthermore, a Stark difference in the thermal evolution of strain in these films depending on substrate materials is observed. It has been demonstrated that not only the differential thermal expansion coefficient of the substrates and the films, but also slippage of the latter during the thermal treatment determines the net strain in the films. The role of the slippage is significantly higher in 2 nm films than in 30 nm films. We believe that the observations provide crucial information on the structural characteristics of the substrate-coupled wafer-scale hBN films for their future use in technology. [ABSTRACT FROM AUTHOR]

We report a detailed ab initio study of two superlattice heterostructures, one component of which is a unit cell of CuPt ordered InSb 0.5 As 0.5 . This alloy part of the heterostructures is a topological semimetal. The other component of each system is a semiconductor, zincblende-InSb, and wurtzite-InAs. Both heterostructures are semiconductors. Our theoretical analysis predicts that the variation in the thickness of the InSb layer in InSb/InSb 0.5 As 0.5 heterostructure renders altered bandgaps with different characteristics (i.e., direct or indirect). The study holds promise for fabricating heterostructures, in which the modulation of the thickness of the layers changes the number of carrier pockets in these systems. [ABSTRACT FROM AUTHOR]

Half-metallic ferromagnetic full Heusler alloy NiFeMnSn is a promising material in spintronic device fabrication as it carries high spin polarization and high Curie temperature (Tc = 405 K). Understanding electronic excitations in a spin-polarized band structure is essential for the further use of this material in spin-based devices and to optimize the spin-dependent electronic structure in related compounds. In this Letter, we report electronic Raman scattering of NiFeMnSn with spectral signatures at a higher wavenumber than expected from the calculated phonon modes of the system. Temperature-dependent Raman spectroscopy demonstrates a non-monotonic behavior of the Raman shift with temperature across Tc. The orbital resolved electronic density of states and electronic band structure for both spin channels of the system, as obtained from the first principles density functional theory calculations, suggest that the observed Raman signatures originate from the Fe d orbital and its splitting due to the crystal field near the Fermi level. Furthermore, a strong magnetic field dependence of the spectral profile is observed. The study not only exhibits electronic Raman scattering in a Heusler alloy compound, which was unexplored to date, but also establishes Raman scattering as a promising probe to study the orbital-resolved partial density of states in the band structure near the Fermi level of a Heusler alloy. [ABSTRACT FROM AUTHOR]

The underlying physics which explains the role of cationic and anionic defect dynamics in determining the catalytic activity and ionic conductivity of aliovalent ion-doped ceria is complex and not yet fully understood. To address this issue, we have investigated the structural evolution of LaxCe1-xO2–δ, where x =0.1, 0.2, 0.3, 0.4, and 0.5. Rietveld refinements of the X-ray diffraction patterns reveal that the crystal structure belongs to a disordered fluorite phase for all samples. We find a systematic increase in the lattice parameter along with a gradual decrease in oxygen site occupancy with the increase in La3+ doping concentration. In Raman spectra of doped compounds, other than well-known crystalline F2g and defect related modes, we observe the presence of an additional Raman mode. The Raman line-shape analysis indicates the non-resonant Fröhlich character of this mode. Temperature dependent Raman measurements demonstrate unique characteristics of this Raman peak. In addition, from the change in relative intensity ratio of the Raman modes related to different types of defect states, we show that vacancies cannot be considered as isolated defects beyond a critical doping level. We propose that the evolution of Raman intensities of defect related modes and Fröhlich mode with the doping level can be used as a marker to determine the role of electron-phonon coupling and anion vacancies in the catalytic activity of doped ceria. Furthermore, by studying the photocatalytic measurements for La3+ doped ceria compounds, we demonstrate that the anion vacancies do not always play a direct role in controlling functional properties. [ABSTRACT FROM AUTHOR]

The problem of finding minimum (local as well as absolute) path lengths joining given points (or terminals) on a plane is known as the Steiner problem. The Steiner problem arises in finding the minimum total road length joining several towns and cities. We study the Steiner tree problem using six-pin soap films. Experimentally, we observe spanning trees as well as Steiner trees partly by varying the pin diameter. We propose a possibly exact expression for the length of a spanning tree or a Steiner tree, which fails mysteriously in certain cases. [DOI: 10.1119/1.3247982]

We describe a simple experiment on the diffraction of monochromatic light by interfering liquid surface waves. The surface wave profile, which acts as a reflection phase grating for incident light, is generated by electrically driven vertical oscillations of two or more slightly immersed exciters. The theoretical intensity of the diffracted light agrees well with observations. In particular, we show the relation between the intensity and the amplitude (height) of the surface wave. Although invisible to the naked eye, the interference of liquid surface waves can be optically detected from the characteristic features of the diffraction pattern. In addition, we can measure the amplitude, wavelength, and phase velocity of the surface wave that propagates along the line joining the exciters. [DOI: 10.1119/1.1870032]

Modern materials science requires efficient processing and characterization techniques for low dimensional systems. Raman and photoluminescence spectroscopy are important nondestructive tools which provide much information about such systems. Commercial Raman spectrometers are expensive. We discuss a less expensive apparatus with assembled collection optics. Studies of Ge nanoparticles, porous silicon (nanowire), carbon nanotubes, and two-dimensional InGaAs quantum layers demonstrate that this apparatus is useful for teaching and research on nanomaterials.

The crystallization of amorphous selenium films was conducted by exposing them to simultaneous irradiation of a 488.0 nm line of Ar+ and a 676.4 nm line of a Kr+ laser by cross-band-gap and sub-band-gap light irradiation. It was observed that the crystallized amorphous selenium films displayed Raman spectra similar to that of trigonal crystalline. Results indicate that the crystallization process is delayed when the material is expose to both of the sources simultaneously.

We embarked on a study of Scandium (Sc) doped (onto Ga site) gallium ferrite (GaFeO3) and found remarkable magnetic properties. In both doped as well as parent compounds, there were three types of Fe3+ions (depending on the symmetry) with the structure conforming to space group Pna21 (Sp. Grp. No. 33) below room temperature down to 5 K. We also found that all Fe3+ions occupy octahedral sites, and carry high spin moment. For the higher Sc substituted sample (Ga1-xScxFeO3:x = 0.3), a canted magnetic ordered state is found. Spin-phonon coupling below Néel temperature was observed in doped compounds. Our results indicated that Sc doping in octahedral site modifies spin-phonon interactions of the parent compound. The spin-phonon coupling strength was estimated for the first time in these Sc substituted compounds. [ABSTRACT FROM AUTHOR]

With more widespread applications of nanotechnology, heat dissipation in nanoscale devices is becoming a critical issue. We study the thermal response of wafer-scale hexagonal boron nitride (hBN) layers, which find potential applications as ideal substrates in two dimensional devices. Sapphire-supported thin hBN films, 2′′ in size and of different thicknesses, were grown using metalorganic vapour phase epitaxy. These large-scale films exhibit wrinkles defects and grain boundaries over their entire area. The shift of phonon mode with temperature is analysed by considering the cumulative contribution of anharmonic phonon decay along with lattice thermal expansion, defect, and strain modulation. The study demonstrates that during heat treatment the strain evolution plays a dominating role in governing the characteristics of the wrinkled thinner films. Interestingly we find that both defects and strain determine the spectral line-width of these wafer-scale films. To the end, from Raman line-width, the changes in phonon lifetime in delaminated and as-grown films is estimated. The results suggest the possibility of a reduction in thermal transport in these wafer-scale films compared to their bulk counterpart. [ABSTRACT FROM AUTHOR]

The correlations between properties of hydrogenated diamondlike carbon films and their Raman spectra have been investigated. The films are prepared by plasma deposition technique, keeping different hydrogen to methane ratios during the growth process. The hydrogen concentration, sp3 content, hardness, and optical Tauc gap of the materials have been estimated from a detailed analysis of their Raman spectra. We have also measured the same parameters of the films by using other commonly used techniques, such as sp3 content in films by x-ray photoelectron spectroscopy, their Tauc gap by ellipsometric measurements, and hardness by microhardness testing. The reasons for the mismatch between the characteristics of the films, as obtained by Raman measurements and by the above mentioned techniques, have been discussed. We emphasize on the importance of the visible Raman spectroscopy in reliably predicting the above key properties of diamondlike carbon films. [ABSTRACT FROM AUTHOR]

Optical properties of nanocrystalline red-emitting phosphor, europium-doped yttria (Y2O3:Eu3+), of average particle size of 15 nm are investigated. The intensity of the strongest emission line at 612 nm is found to be highest in the nanocrystalline sample with 4 at. wt % of europium. The narrow electronic emission spectrum suggests a crystalline surrounding in this nanomaterial. We have estimated the strength of the crystal-field parameter at the dopant site, which plays a crucial role in determining the appearance of the intense emission line. The equilibrium temperature of this system has also been calculated from the intensity ratio of Stokes and anti-Stokes Raman scattering. Though known for the bulk samples, our approach and consequent results on the crystalline nanomaterial of Y2O3:Eu3+ provide a unique report, which, we believe, can be of considerable significance in nanotechnology. [ABSTRACT FROM AUTHOR]

An investigation on the optical properties of nanocrystalline red-emitting phosphor, europium-doped yttria [Y2O3:Eu(super 3+)], of average particle size of 15 nm is presented. The intensity of the strongest emission line at 612 nm is found to be highest in the nanocrystalline sample with 4 at. wt% of europium.

Rare earth indates are an interesting class of compounds with rich crystallography. The present study explores the crystallographic phases observed in REInO3 (RE: La–Yb) systems and their dependence on synthesis routes and annealing temperature. All REInO3 compositions were synthesized by a solid state route as well as gel-combustion synthesis (GC) followed by annealing at different temperatures. The systems were well characterized by powder XRD studies and were analysed by Rietveld refinement for the structural parameters. The cell parameters were observed to decrease in accordance with the trend in ionic radii on proceeding from lighter to heavier rare earth ions. Interestingly, the synthesis route and the annealing temperature had a profound bearing on the phase relationships observed in the REInO3 series. The solid state synthesized samples depicted an orthorhombic phase (Pbnm) field for LaInO3 to SmInO3, followed by a hexagonal-type phase (P63cm) for GdInO3 to DyInO3. However, the phase field distribution was greatly influenced upon employing gel-combustion (GC) wherein both single-phasic hexagonal and orthorhombic phase fields were found to shrink. Annealing the GC-synthesized compositions to still higher temperatures (1250 °C) further evolved the phase boundaries. An important outcome of the study is observance of polymorphism in SmInO3 which crystallized in the hexagonal phase when synthesized by GC and orthorhombic phase by solid state synthesis. This reveals the all-important role played by synthesis conditions. The existence and energetics of the two polymorphs have been elucidated and discussed with the aid of theoretical studies. [ABSTRACT FROM AUTHOR]

In this article, we compare the excitation wavelength dependence of the polarization anisotropy (ρ) of an internal field induced Raman scattering signal in individual bare InAs and InAs/GaSb core-shell nanowires. The measured value of ρ of the Raman scattering intensity for InAs/GaSb core-shell nanowires has a minimum at ∼500 nm, while for the bare InAs nanowire, the value of ρ monotonically increases over the same range of wavelengths. We have modeled the scattering intensities of both systems by considering the joint role of Raman tensor components and confinement of electromagnetic radiation inside the nanowire at two orthogonal polarization configurations of the electromagnetic radiation. The theoretical results allow us to understand that the observed behavior of ρ is related to the nanowire geometry and to the difference in the wavelength dependence of the dielectric constants of InAs and GaSb. This work shows the possibility of manipulating the polarization anisotropy by selecting suitable diameters and materials for the core and the shell of the nanowire. We also report a six-fold increase in Raman scattering intensity due to the GaSb shell on InAs nanowires. [ABSTRACT FROM AUTHOR]

Raman spectroscopy and Hall measurements have been carried out to investigate the differences in near-surface charge carrier modulation in high energy (~100 MeV) silicon ion (Si8+) and oxygen ion (O7+) irradiated n-GaAs. In the case of O ion irradiation, the observed decrease in carrier concentration with increase in ion fluence could be explained in the view of charge compensation by possible point defect trap centers, which can form because of elastic collisions of high energy ions with the target nuclei. In Si irradiated n-GaAs one would expect the carrier compensation to occur at a fluence of 2.5 × 1013 ions/cm2, if the same mechanism of acceptor state formation, as in case of O irradiation, is considered. However, we observe the charge compensation in this system at a fluence of 5 × 1012 ions/cm2. We discuss the role of the complex defect states, which are formed because of the interaction of the primary point defects, in determining carrier concentration in a Si irradiated n-GaAs wafer. The above results are combined with the reported data from the literature for high energy silver ion irradiated n-GaAs, in order to illustrate the effect of both electronic and nuclear energy loss on trap creation and charge compensation. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We study band alignment in wurtzite/zinc-blende polytype InAs heterostructured nanowires using temperaturedependent resonance Raman measurements. Nanowires having two different wurtzite fractions are investigated. Using visible excitation wavelengths in resonance Raman measurements, we probe the electronic band alignment of these semiconductor nanowires near a high-symmetry point of the Brillouin zone (E1 gap). The strain in the crystal structure, as revealed from the shift of the phonon mode, explains the observed band alignment at the wurtzite/zinc-blende interface. Our experimental results are further supported by electronic-structure calculations for such periodic heterostructured interface. [ABSTRACT FROM AUTHOR]

The article presents a mapping of the residual strain along the axis of InAs/InSb heterostructured nanowires. Using confocal Raman measurements, we observe a gradual shift in the transverse optical phonon mode along the axis of these nanowires. We attribute the observed shift to a residual strain arising from the InAs/InSb lattice mismatch. We find that the strain is maximum at the interface and then monotonically relaxes towards the tip of the nanowires. We also analyze the crystal structure of the InSb segment through selected area electron diffraction measurements and electron diffraction tomography on individual nanowires. [ABSTRACT FROM AUTHOR]

In this article we revisit the much-studied behavior of self-assembled aggregates of gold colloidal particles. In the literature, the electrostatic interactions, van der Waals interactions, and the change in free energy due to ligand-ligand or ligand-solvent interactions are mainly considered to be the dominating factors in determining the characteristics of the gold aggregates. However, our light scattering and imaging experiments clearly indicate a distinct effect of light in the growth structure of the gold colloidal particles. We attribute this to the effect of a non-uniform distribution of the electric field in aggregated gold colloids under the influence of light. [ABSTRACT FROM AUTHOR]

In this article, we report the structural phase transitions in Bi4 V2 O11 as observed from temperature-dependent Raman scattering and X-ray diffraction measurements. Four different types of highly disordered coordination polyhedra around the vanadium atoms with large dispersion of V-O bond lengths are observed in Bi4 V2 O11 at ambient temperature. The observed V-O bond lengths could be grouped into two categories, viz. shorter <1.7 Å and longer >1.7 Å. The Raman modes of Bi4 V2 O11 could be assigned to vibration of these bonds and V-O-V linkages. We could correlate the difference in degree of anharmonicity of the phonon modes with temperature to differences in V-O bond strength. The local structure of vanadium-oxygen network in Bi4 V1.8 Cu0.2 O10.7 was also obtained by similar studies. The effect of highly disordered anion sublattice in the doped compound is reflected in the broadening of the Raman modes. [ABSTRACT FROM AUTHOR]

The interaction of l-tryptophan (Trp) with silver colloids was investigated at between pH values of 6.11 and 10.6 of the sol. The measurements of surface-enhanced Raman bands of Trp in the colloidal solution indicate the evolution of interaction between the metal particles and the molecules with increasing pH values for the sol. The experimental observations were explained using the estimated atomic charge distribution in the zwitterionic and anionic forms of the residue, obtained by density functional theory calculations. The variation in the ratio of the spectral intensities of the Fermi resonance bands with the pH reflects the effect of the colloidal environment on Trp. The results obtained can be used as a marker for describing the nature of the interaction of silver colloids with the specific terminus of the residue, at varying pH environments. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]