A. K. Bhattacharya

X-ray photoelectron spectroscopy and spectral transmittance study of stoichiometry in sputtered vanadium oxide films

Abstract The growth of magnetron sputtered vanadium oxide thin films has been investigated by means of X-ray photoelectron spectroscopy and spectral transmittance in the region from 350 to 1100 nm. It is shown that films stoichiometry is dependent on film thickness as well as oxygen partial pressure. Beyond a critical thickness of ∼ 250 nm, the films are completely stoichiometric V 2 O 5 independent of the oxygen partial pressure. At low thickness, the films are a mixtured of V 2 O 3 , VO 2 and V 2 O 5 in varying proportions depending on the oxygen pressure. The presence of the non-stoichiometry has been correlated with changes in the spectral transmittance. It has also been shown that the mixed oxide phase is stable over a large regime of thickness and oxygen partial pressure during processing and could have very useful optical properties. A mechanism for the observed stoichiometry behaviour has also been proposed.

Structure and stability of copper clusters: A tight-binding molecular dynamics study

In this paper we propose a tight-binding molecular dynamics with parameters fitted to first-principles calculations on the smaller clusters and with an environment correction, to be a powerful technique for studying large transition-metal/noble-metal clusters. In particular, the structure and stability of Cu{sub n} clusters for n=3-55 are studied by using this technique. The results for small Cu{sub n} clusters (n=3-9) show good agreement with ab initio calculations and available experimental results. In the size range 10{<=}n{<=}55 most of the clusters adopt icosahedral structure which can be derived from the 13-atom icosahedron, the polyicosahedral 19-, 23-, and 26-atom clusters, and the 55-atom icosahedron, by adding or removing atoms. However, a local geometrical change from icosahedral to decahedral structure is observed for n=40-44 and return to the icosahedral growth pattern is found at n=45 which continues. Electronic magic numbers ( n=2, 8, 20, 34, 40) in this regime are correctly reproduced. Due to electron pairing in highest occupied molecular orbitals (HOMOs), even-odd alternation is found. A sudden loss of even-odd alternation in second difference of cluster binding energy, HOMO-LUMO (LUMO, lowest unoccupied molecular orbital) gap energy and ionization potential is observed in the region n{approx}40 due to structural change there. Interplay betweenmorexa0» electronic and geometrical structure is found.«xa0less

Variable optical absorption edge in ion beam sputtered thin ytterbium oxide films

Abstract Thin films of ytterbium oxide have been deposited by ion beam sputtering from the oxide target on to fused silica substrates. The films that were in the thickness range 50 to 150 nm were characterised for spectral transmittance and reflectance and the refractive index, extinction coefficient and optical absorption edge. It is demonstrated that all these quantities are extremely sensitive to variation in thickness of the films. The refractive index increases with thickness while the extinction coefficient decreases with increase in thickness. The thickest films (∼150 nm) have a refractive index of 1.75 at 600 nm which decreases to 1.55 for the 50 nm thick film. Most significantly the optical absorption edge varies from 4.85 to 4.63 eV over the same range of thickness. The observed behaviour has been fitted in to the single effective oscillator model and it is shown that this model predicts an increase in absorption edge with decreasing thickness. It is also shown, based on this model, that the variation in refractive index and absorption edge are inter-related. The dispersion energy, which is a measure of the strength of interband transitions increases with increase in thickness.

Processing and size effects on the optical properties of sputtered oxide thin films

Abstract The variation in refractive index, extinction coefficient and optical absorption edge with thickness of magnetron sputtered vanadium and niobium oxide films and ytterbium oxide films deposited by both magnetron sputtering (MS) and ion beam sputtering (IBS) has been investigated. The thickness of the films was varied in the range 50–400 nm. In all the cases it is shown that with increase in thickness the extinction coefficient and the absorption edge decreased, while the refractive index increased. Typically, for the MS ytterbia films the variation was between 1.6 and 1.75 whereas it was between 1.7 and 1.8 for the IBS films. The absorption edge varied from 4.5 to 4.7 eV for ytterbia MS thin films while it was between 4.6 and 4.8 eV for the IBS films. It is demonstrated that independent of the material, below a critical thickness the variations in refractive index, extinction coefficient and absorption edge are interrelated. It is therefore postulated that independent of the processing technique a material dependent critical thickness has to be achieved for the films to exhibit bulk like behaviour and the absorption edge and refractive index to become independent of each other.

Spectral emissivity of ytterbium oxide-based materials for application as selective emitters in thermophotovoltaic devices

Abstract The spectral emissivity of Yb 3+ in a series of compounds has been investigated to study the effects of crystal structure type, chemical environment, ytterbium concentration, impurity concentration and temperature on the spectral radiant intensity of the selective emitter peak and emitted power from the material. A figure of merit has been defined which enables the compounds to be ranked for application in a practical thermophotovoltaic energy conversion device. It is shown that significant selective emission can be achieved from compounds in which the Yb 3+ concentration is as low as 10xa0mol%. Apart from pure Yb 2 O 3 , the compounds Yb 3 Al 5 O 12 , YbNbO 4 and Y 0.9 Yb 0.1 O 1.5 are found to have emission spectra suitable for efficient matching to silicon photovoltaic cells.

A study of nanocrystalline CeO2/PrOx optoionic thin films: temperature and oxygen vacancy dependence

Abstract The structure and optical constants of praseodymia doped ceria thin films are reported with and without oxygen annealing. These films were prepared using an inorganic sol-gel route and crystallized into the cubic fluorite structure well below 400°C, with cell constants altering as a function of dopant concentration and oxygen treatment/vacancy concentration. The films were also nanocrystalline, continuous and highly transparent in the region between 350–1500 nm. Optical transmission decreased with increased crystallite size due to scattering, below 600 nm. The refractive indices of the films at 800 nm in the dispersion free region were shown to vary in direct proportion to the lattice parameter/vacancy concentration and temperature treatment, and varied between 1.77 and 2.26. It is therefore proposed that they should be suitable materials for application as transparent ion intercalation films and optoionic smart windows.

Low temperature preparation of orthoferrite thin-films by an inorganic sol–gel process

Abstract Aqueous precursor sols of orthoferrites were prepared by room temperature processing of inexpensive commercially available metal salts. Crystalline thin-films of orthoferrites of general formula, LnFeO3(Ln=La, Nd, Sm, Gd, Dy, Er, Yb and Y) were prepared by dip-coating with the precursor sol and heating at 650°C. All the films showed >80% transmittance independent of the rare earth ion. The optical band-gap was 3.1 eV and the refractive index was 1.61±0.02 independent of the rare earth ion. The films exhibited uniformity in thickness, mechanical and thermal integrity upon heating.

Temperature and ionic size dependence on the structure and optical properties of nanocrystalline lanthanide doped zirconia thin films

Abstract Sols of lanthanide-doped zirconia with the general formula Zr0.90Ln0.10O2−x were prepared by an inorganic sol–gel method. The sols were characterized for particle size distribution, and the gels and heated gels were characterized for structure crystallite size and lattice parameter. Thin films of these materials were deposited on quartz substrates, and were crystalline, continuous and single phase as-deposited, with a cubic fluorite structure as shown by X-ray diffraction. The films were annealed to 600 and 1050°C after deposition and found to be transparent in the region between 400 and 1100 nm, the crystalline structure becoming tetragonal at 1050°C. The refractive index increased with increase in annealing temperature and decreased with ionic size of the dopant, varying between 1.53 for the largest cation, and 1.65 for the smallest cation at 600°C, and between 1.77 and 1.90 over the same range at 1050°C. Optical band gap calculations also showed a similar ionic size and temperature dependence with the optical band gap varying from 5.55 to 5.70 eV.

ELECTRIC-FIELD-INDUCED NANOSTRUCTURING OF METALLIC THIN FILMS

The nanostructuring of thin Ni and In films by postdeposition electric field treatment is presented. Electric-field-induced nanostructuring effects, such as organization into an array of nanoparticles, formation of nanowires and transition from amorphous to nanocrystalline states, are demonstrated. It is further shown that the nanostructures can be manipulated by simply changing the direction of the applied field. Control parameters include separation of field probes (2–5 mm), duration (20–60 s) and direction of the field applied, and thickness (10–100 nm). High fields cause field-induced emission in the films. It is observed that while the microstructural reconstruction of the surfaces can be manipulated, the amorphous–crystalline transition is irreversible.

Structure and optical properties of nanocrystalline yttria doped ceria thin films

Thin films of Ce1-xYxO2-y where x ranges from 0 to 0.5 have been coated onto glass substrates using an inorganic sol–gel approach at low temperature. The lattice parameters from powder diffraction measurements were calculated and shown to be very close to those previously reported. Crystallite size measurements indicated that the films were nanocrystalline, the size decreasing as a function of dopant concentration. The films are transparent in the region 500 to 1500 nm with very low optical losses. The film refractive index is dependent on the dopant concentration and peaks at an yttria concentration of x=0.25 after treatment at 450°C, with a value of 1.79, which on increasing the yttria concentration to x=0.50 decreases to 1.65 in the dispersion free region. The optical band gap is also dependent on the dopant concentration and is in the range 3.2 to 3.0 eV.