S. M. Shivaprasad

Surface-modified CuO layer in size-stabilized single-phase Cu2O nanoparticles

Activated reactive evaporation has been used to grow copper oxide nanoparticles in the size range of 8–100 nm. X-ray diffraction spectra clearly show the presence of a single Cu2O phase. Detailed x-ray photoelectron spectroscopy studies show an increase in the ionicity of the Cu2O system with decreasing particle size. Depth profiling and finger printing of x-ray photoelectron spectra reveal that the Cu2O nanoparticles are capped with a CuO surface layer of thickness ≈1.6 nm. This study strongly suggests that the stabilization of the cubic Cu2O nanophase is enhanced by the formation of a CuO surface layer.

Modifying the nanocrystalline characteristics—structure, size, and surface states of copper oxide thin films by high-energy heavy-ion irradiation

In the present study, x-ray diffraction, Raman spectroscopy, spectroscopic ellipsometry, photoluminescence, and x-ray photoelectron spectroscopy techniques were used to study the effect of 120 MeV 107Ag9+ ion irradiation on nanocrystalline Cu2O thin films grown by the activated reactive evaporation technique. The influence of dense electronic excitations during ion irradiation on the structural and optical properties of the Cu2O thin films was studied. Experimental results demonstrate that the phase and the size of nanocrystallites in the Cu2O thin films as well as associated surface states can be tailored by controlling ion fluence. The Cu2O higher symmetry cubic phase is observed to be quite stable under a higher temperature and irradiation-induced thermal spikes, which accompanies ion irradiation.

Electrochromic Nanostructured Tungsten Oxide Films by Sol-gel: Structure and Intercalation Properties

As-deposited sol-gel derived amorphous tungsten oxide films transform into nanostructured films with an interconnected framework of grains and pores and a dominant triclinic crystalline phase upon annealing at 250 degrees C. Transmission electron microscopy and scanning electron microscopy images clearly reveal the annealing-induced microstructural evolution for the film. Subsequent to lithium intercalation, the film annealed at 250 degrees C shows quasi-reversible structural changes, as ascertained by X-ray diffraction and Fourier transform infrared spectral data. Dynamic transmission modulation for film revealed a high optical modulation of 72% (gimel=650nm) and a coloration efficiency maximum of 132 cm(2) C-1 at 800 nm under a lithium intercalation level of x = 0.20. X-ray photoelectron spectroscopy of the W 4f core levels demonstrated a progressive increase in the W5+ content at the expense of W6+ proportion as the insertion coefficient was raised from 0 to 0.25, with 0.20 as the threshold value above which the W5+ content exceeds the W6+ proportion. A new W4+ state also appears which acts to lower the coloration efficiency for x >= 0.22. The presence of charged oxygen interstitials in the vicinity of electrochemically active tungsten sites is also responsible for the coloration efficiency decline at high ion insertion levels.

Size dependence of core and valence binding energies in Pd nanoparticles: Interplay of quantum confinement and coordination reduction

An analysis is given of the electronic structure of Pd nanoparticles synthesized by inert gas evaporation technique. A study of the effect of size on various core and valence electrons in Pd nanoparticles reveals a varied dependence of binding energy of electrons in different electronic levels. The shift in the Pd x-ray photoelectron spectroscopy 4d valence band centroid is more than the core level shift. The results of the present study provide a direct evidence of interplay of quantum confinement (a size effect) and coordination reduction (a surface effect).

Enhancement of luminescent properties of ZnS:Mn nanophosphors by controlled ZnO capping

Results of a method is presented for synthesizing ZnS:Mn nanoparticles capped in situ by ZnO. Analysis of Raman spectra and x-ray photoelectron spectra results have reinforced claim of the formation of ZnO capping layer on the surface of ZnS:Mn nanoparticles. Raman spectra results also showed presence of stress at an optimum ZnO capping thickness. In brief, the only variation within samples is in their ZnO capping thickness. Phase formation was analyzed and confirmed from powder x-ray diffraction. ZnS:Mn particle size is about 4nm. The change in photoluminescent properties with ZnO capping thickness variation is presented. It is shown that the variation in ZnO thickness and the resultant stress leads to an enhanced photoluminescence intensity/efficiency of nano-ZnS:Mn.

Universal metal-semiconductor hybrid nanostructured SERS substrate for biosensing.

We demonstrate here a novel high surface area GaN nanowall network substrate with plasmonic Ag nanodroplets, that can be employed as a highly sensitive, reproducible, and charge independent SERS substrate. The uniformity of the size and distribution of the Ag droplets and the absence of linker ligands result in large near-field intensity, while the GaN nanowall network morphology provides multiple reflections for signal enhancement. FDTD calculations simulate the observed hot-spot distribution and reiterate the higher performance of this hybrid substrate over conventional ones. Our studies on oppositely charged proteins provide a proof of concept for employing this as a versatile charge independent label free SERS substrate for trace biomolecule detection.

Microwave plasma oxidation of gallium nitride

Abstract Low temperature oxidation of gallium nitride epilayer by microwave oxygen plasma treatment has been characterized by secondary ion mass spectroscopy and X-ray photoelectron spectroscopy (XPS). In the initial stage the oxide layer grows almost linearly with time while after approximately a thickness of 10 nm the growth rate saturates. The chemical shift of 1.2 eV in the Ga 2p and 3d peak in XPS data is observed upon oxidation, indicating the formation of Ga2O3 on the GaN surface.

Nitrogen flux induced GaN nanostructure nucleation at misfit dislocations on Al2O3(0001)

The work demonstrates the dominant role of nitrogen flux rate on GaN nanostructure formation on bare Al2O3(0001). In nitrogen rich conditions, wurtzite c-oriented GaN nanowall honeycomb network is formed as strain relaxation pathway of nucleation at edge dislocations. A specific nitrogen flux rate in a plasma assisted molecular beam epitaxy growth is necessary for fixed Ga flux and substrate temperature to form columnar self assembled nanostructures. It is argued that kinetically hindering diffusion of Ga adatoms and the low sticking coefficient of r and m planes of nanowalls promote 1-dimension nanocolumn formation at screw dislocations formed at the GaN-Sapphire interface.

The formation of MnSi(111) interface at room and high temperatures

Abstract The formation of a Mn Si (111) interface is studied by depositing submonolayer to several monolayer coverages θ of Mn at controlled rates and at different substrate and annealing temperatures. The observation of various surface phase formations, their transitions, and their dependence on deposition parameters are probed by Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and electron energy loss spectroscopy (ELS). Deposition of Mn at very low rates (0.15 ML min −1 ) resulted in the formation of epitaxial structures even at room temperature (RT). It is observed for the first time that up to 1 ML, the low rate of arrival (deposition rate −1 ) of Mn atoms at the Si(111) surface facilitates the growth of a Mn Si (111)-7 × 7 surface phase. Mn atoms arriving thereafter accumulate in the layer-by-layer (Frank-van der Merwe) mode resulting in the 1 × 1 epitaxial growth of Mn at least up to 2–3 ML at RT. Annealing at 350°C of the RT deposited Mn (1 θ 3 ML) Mn Si (111) system results in a sharp √3 × √3 phase, which corresponds to the formation of manganese silicide as observed by ELS. It is found that using the 1 × 1 surface phase as a template results in high quality epitaxial √3 × √3 silicide formation. Deposition at low rates onto heated (350°C) Si(111) leads to Si(111)Mn 7 × 7 surface phase with the excess adatoms forming islands on the surface (Stranski-Krastanov mode).

Size-induced stability and structural transition in monodispersed indium nanoparticles

The present study reports the stability and the physical significance of the size-induced crystallographic structural transition in the gas-phase synthesized monodispersed indium nanoparticles. Transmission electron microscopy and x-ray photoelectron spectroscopy studies reveal that the formation of a thin oxide shell results in enhanced stability of indium nanoparticles. These results also show a size-induced structural transition from the bulk tetragonal to face-centered-cubic structure, which is attributed to an increase in the binding energy of core electrons of indium nanoparticles due to quantum confinement effects and the presence of a thin oxide shell.