Optical bandgap

We report the properties of thermally evaporated Ge 11.5 As 24 Se 64.5 chalcogenide films ion implanted at energies of 2.25MeV with Erbium at concentrations up to 0.4 mol%. The effect of post implant annealing on the refractive index of the films and on the 4 I 13/2 → 4 I 15/2 Er transition was studied. Photoluminescence was found to increase significantly and a lifetime of 1.35 ms was obtained in films annealed at 180°C. Different apparent lifetimes for the 4 I 13/2 → 4 I 15/2 transition were obtained for 980nm and 1470nm pumps and the origins of this phenomenon are discussed.

We report on the bandgap variation in thin films of BxGa1−xN grown on AlN/sapphire substrates using metal-organic vapor phase epitaxy. Optical transmission, photoluminescence, and x-ray diffraction were utilized to characterize the materials’ properties of the BxGa1−xN films. In contrast to the common expectation for the bandgap variation, which is based on the linear interpolation between the corresponding GaN and BN values, a significant bowing (C=9.2±0.5 eV) of the bandgap was observed. A decrease in the optical bandgap by 150 meV with respect to that of GaN was measured for the increase in the boron composition from 0% to 1.8%.

The usage of semiconductor nanostructures is highly promising for boosting the energy conversion efficiency in photovoltaics technology, but still some of the underlying mechanisms are not well understood at the nanoscale length. Ge quantum dots (QDs) should have a larger absorption and a more efficient quantum confinement effect than Si ones, thus they are good candidate for third-generation solar cells. In this work, Ge QDs embedded in silica matrix have been synthesized through magnetron sputtering deposition and annealing up to 800°C. The thermal evolution of the QD size (2 to 10 nm) has been followed by transmission electron microscopy and X-ray diffraction techniques, evidencing an Ostwald ripening mechanism with a concomitant amorphous-crystalline transition. The optical absorption of Ge nanoclusters has been measured by spectrophotometry analyses, evidencing an optical bandgap of 1.6 eV, unexpectedly independent of the QDs size or of the solid phase (amorphous or crystalline...

There has been renewed interest in solar concentrators and optical antennas for improvements in photovoltaic energy harvesting and new optoelectronic devices. In this work, we dielectrophoretically assemble single-walled carbon nanotubes (SWNTs) of homogeneous composition into aligned filaments that can exchange excitation energy, concentrating it to the centre of core-shell structures with radial gradients in the optical bandgap. We find an unusually sharp, reversible decay in photoemission that occurs as such filaments are cycled from ambient temperature to only 357 K, attributed to the strongly temperature-dependent second-order Auger process. Core-shell structures consisting of annular shells of mostly (6,5) SWNTs (E g = 1.21 eV) and cores with bandgaps smaller than those of the shell (E g = 1.17 eV (7,5)-0.98 eV (8,7)) demonstrate the concentration concept: broadband absorption in the ultraviolet-near-infrared wavelength regime provides quasi-singular photoemission at the (8,7) SWNTs. This approach demonstrates the potential of specifically designed collections of nanotubes to manipulate and concentrate excitons in unique ways.

Hydrogenated amorphous siliconycarbon films (a-Si-C:H) are deposited from a silane and acetylene gas mixture by the catalytic chemical vapour deposition (Cat-CVD) technique. It is observed that under certain conditions of total gas pressure and filament temperature (T ), the optical bandgap varies non-linearly with the acetylene to silane (C H ySiH ) ratio, having a maximum F 2 2 4 value of 3.6 eV for a C H ySiH ratio G0.8. However, the deposition rate drastically reduces with an increase in acetylene 2 2 4 fraction. FTIR spectra indicate that the total hydrogen content is lower compared to samples deposited by PECVD using similar gas mixtures, with hydrogen being preferentially attached to carbon rather than silicon atoms. The photoluminescence (PL) spectra of these films show PL in the visible spectral region at room temperature. The films with larger bandgap ()2.5 eV) exhibit PL at room temperature, with the emission having peak energy in the range 2.0-2.3 eV.

Thin films of arsenic triselenide doped with silver were prepared by thermal evaporation of the bulk-quenched materials. Structural investigations have been carried out by using X-ray diffraction and X-ray fluorescence. The differential scanning calorimetry is used to measure the glass transition temperature of the samples under investigation. The Swanepoel method based on the use of the maxima and minima in the interference fringes has been successfully utilized for accurate determination of the film thickness and optical parameters. The technique used took into consideration the non-uniform nature in the film thickness. Using the exact calculation of the film thickness the real refractive index, the extinction coefficient, the optical bandgap energy and the oscillator and dispersion energies were obtained as a function of the silver contents. The DC conductivity was found to depend on the silver content.

Polycrvstalline thin films ofundoped CdS and In doped CdS (i.e. CdS:In) have been deposited by chemical bath on glass slides and high purity silicon wafers. The cflcct of different processing conditions (e.g. substrate temperature, doping and air annealing) on film growth, optical transmittance and electrical properties were correlated with the film microstrucutre. The mechanism of chemical bath deposition ofCdS thin films from the ammonia-thiourea system is studied. The influence of reaction parameters (i.e. concentration of reactants and pH) on film growth were determined and modeled. Thin film growth is thermally activated with an energy::::: 5 x IOn. eV•mor 1 where it initiates a sequential reaction mechanism. A kinetic study is presented resulting in the formulation of a growth rate formula. CdS films exhibited high resistivities typically ~ I 0 5 -I 0 6 O•cm that decreased with increasing [Cd] and ,TU] concentrations. Optical band gap energies of the mixture hexagonal and cubic structures were::::: 2.46eV. The band energies were found to be proportional to [Cd] concentrations and inversely proportional to [TU] concentrations. CdS:ln films were furmed by in-situ chemical doping with In ions. The resulting films revealed a more organized crystal structure than the undoped films. A comparison of the optical transmittance spectra for undoped and doped films indicated improved film crystallinity and a marginal impurlty absorption band at In dopant concentrations< 10 19 atoms•cm•'. Then-type conductivities were typically-0.1 n•'•cm•' (as deposited) and-10 n•'•cm•' (air annealed at 200'C) band gaps differ in the range of2.46 to 2.37 eV. Films annealed h1 air at < 200°C exhibit a fast photoresponse, and a photoconductivity to dark current ratio of~ I 0''. Annealed films show an optical transmittance increase of about 8 -12% at the band edge and caused the band edge and a shift in the band edge to higher wavelengths. Optical band gaps of the films were found to decrease from 2.46 cV for (as deposited) to 2.38 eV for films annealed at 395°C for I hr. Film resistivity decreased by a factor of-lOs as a result of impurity phases and improved crystal orientation. The higher conductivity of the air annealed films were attributed to the presence of the conducting CdO phase. XRD studies of the annealed films confirmed the presence of a mixture of CdO and CdSO, related phases.

There has been renewed interest in solar concentrators and optical antennas for improvements in photovoltaic energy harvesting and new optoelectronic devices. In this work, we dielectrophoretically assemble single-walled carbon nanotubes (SWNTs) of homogeneous composition into aligned filaments that can exchange excitation energy, concentrating it to the centre of core-shell structures with radial gradients in the optical bandgap. We find an unusually sharp, reversible decay in photoemission that occurs as such filaments are cycled from ambient temperature to only 357 K, attributed to the strongly temperature-dependent second-order Auger process. Core-shell structures consisting of annular shells of mostly (6,5) SWNTs (E g = 1.21 eV) and cores with bandgaps smaller than those of the shell (E g = 1.17 eV (7,5)-0.98 eV (8,7)) demonstrate the concentration concept: broadband absorption in the ultraviolet-near-infrared wavelength regime provides quasi-singular photoemission at the (8,7) SWNTs. This approach demonstrates the potential of specifically designed collections of nanotubes to manipulate and concentrate excitons in unique ways.

Thin films of (WO 3 ) 1 Àx -(Fe 2 O 3 ) x composition were deposited by thermal evaporation on glass substrates and then all samples were annealed at 400 -C in air. Optical properties such as transmittance, reflectance, and optical bandgap energy of the ''as deposited'' and the annealed films were studied using ultraviolet -visible spectrophotometry. It was shown that the annealing process did not substantially change the optical transmittance and reflectance of all the films except the films having x = 0.75. By increasing Fe 2 O 3 content in the films from x = 0 to x = 0.75, optical bandgap energy decreased from 3.4 to about 1.3 eV and from 3.1 to 2.1 eV for the ''as deposited'' and the annealed samples, respectively. Moreover, using X-ray photoelectron spectroscopy, we have shown that the ''as deposited'' (WO 3 ) 0.25 -(Fe 2 O 3 ) 0.75 films contained Fe 3 O 4 , and it converted to Fe 2 O 3 after the annealing process. Using atomic force microscopy, it was found that the grain size and surface roughness of the annealed films were greater than the ''as deposited'' ones. In addition, for the annealed films with intermediate x's, we have observed a smoother surface with smaller grain size as compared to films with the other x's.

A series of step‐ladder copolymers based on thiophene–phenylene–thiophene SL1‐SL3 and thiophene–naphthylene–thiophene SL4 repeat units with varying lengths of the oligothiophene segment has been designed and synthesized via a microwave‐assisted Stille‐type cross‐coupling reaction followed by a polymer‐analogous cyclization reaction. The optical properties of the step‐ladder copolymers have been investigated in detail, in particular at low temperature and in the solid‐state. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7342–7353, 2008

Ultrasonic mist chemical vapour deposition (UM-CVD) system has been developed to prepare ZnO nanopowder. This is a promising method for large area deposition at low temperature inspite of being simple, inexpensive and safe. The particle size, lattice parameters and crystal structure of ZnO nanopowder are characterized by in situ high temperature X-ray diffraction (XRD). Surface morphology of powder was studied using transmission electron microscopy (TEM) and field emission electron microscope (FESEM). The optical properties are observed using UV-visible spectrophotometer. The influence of high temperature vacuum annealing on XRD pattern is systematically studied. Results of high temperature XRD showed prominent 100, 002 and 101 reflections among which 101 is of highest intensity. With increase in temperature, a systematic shift in peak positions towards lower 2θ values has been observed, which may be due to change in lattice parameters. Temperature dependence of lattice constants under vacuum shows linear increase in their values. Diffraction patterns obtained from TEM are also in agreement with the XRD data. The synthesized powder exhibited the estimated direct bandgap (E g ) of 3⋅43 eV. The optical bandgap calculated from Tauc's relation and the bandgap calculated from the particle size inferred from XRD were in agreement with each other.

The growth of hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (a-SiN:H) films was studied by plasma enhanced chemical vapor deposition (PECVD) method. 10% silane (SiH4) diluted in'hydrogen (H,) gas and 100% ammonia (NH,) gas were used as gas sources. The optical band-gap and deposition rate of a-Si:H film were found varied from 1.70 to 1.95 eV and 5 1 to 84 Ao/min, respectidly, when the SiHl gas flow rate varied from 5 to 11 sccm. The widest optical bandgap of a-SiN:H films whichis of 3.69 eV and lowest dark conductivity of 1. 0 7~1 0-~' Scni-' were obtained at NH3 gas fraction of 60% at SiH, flow rate 7 sccm. It is also shown that wider optical bandgap of a-SiN:H can be obtained at the flow rate of SiH4 gas of 5 sccm where its value reaches 3.97 eV at NH3 gas fraction of 25%, whilst its dark conductivity reaches lower value of 1.05x10-12 Scm-I. The application of the films as an insulator gate in the thin film transistor (TFT) device was also studied. The lowest dark conductivity of the a-SiN:H film resulted in a better device thereshold voltage.

Gallium antimonide (GaSb) which is a narrow band gap compound semiconductor has received attention because of its potential applications in optoelectronic devices. In the present work, p-type GaSb wafers of h1 0 0i orientation were irradiated with 70 MeV 56 Fe ions at fluences varying from 1 • 10 12 to 1 • 10 14 ions cm À2. Mid-infrared and Far-infrared Fourier Transform (FT) measurements were carried out to investigate the optical properties of as irradiated and vacuum annealed samples. Mid-infrared Fourier Transform study revealed that the optical absorption of the irradiated samples increases with increasing ion fluence due to increase in irradiation-induced defects. The band gap energy determined from the infrared spectra was found to change from 0.65 to 0.62 eV while for non-irradiated GaSb wafer the corresponding estimate was 0.67 eV. The density of the carrier estimated from the plasma frequency (x p) was found to vary from 2.05 • 10 18 to 1.9 • 10 18 cm À3. The samples annealed in vacuum (10 À6 mb) over the temperature range 100-600°C showed the significant damage recovery.

In this paper, spectral properties of the Fibonacci-class one-dimensional quasi-periodic structures, F C J (n), as an important optical structure are investigated. Analytical relations for description of the spectral properties of F C J (n) are used. Fast Fourier Transform (FFT) for investigation of the spectral properties of these structures is proposed. FFT spectrum of the Fibonacci-class one-dimensional quasi-periodic structures contains peaks that are equivalent to photonic bandgaps or multiband reflection filter. Based on the proposed relations and FFT simulation results, the optical bandgap and other properties of these structures are studied. In this paper, the effects of the optical and geometrical parameters on optical properties of the Fibonacci quasi-periodic structures are considered. Our proposed relations show that the spectral contents of the Fibonacci-class onedimensional quasi-periodic structures have two main terms including the low and high frequency parts. Our results illustrate that the high frequency term depends up on the class order, n, and the width of the layer B, d b , while the low frequency term depends on the width

Silicon nanoparticles with diameters ranging from 3 to 50 nm were prepared by thermal evaporation. The nanoparticles showed visible light emissions from 5000 A to 9000 A, with peak intensity at 8000-8200 A, when excited with a He-Cd laser. The intensity increased with decreasing particle size. Based on the photoluminescence (PL) characteristics, it is suggested that the PL for particles larger than 9 nm is surface state-induced, while the PL for those smaller than 9 nm is ascribed to quantum confinement effects.

The present paper reports the electrical conduclivil) and optical handgap measurements of a-Si,S:H films in lhe alloy range (S/St ratio 0.51,10-" to 4.07* I0-')as a function of depo,,dti,.m temperature. The a-Si.S:lt samples were prepared by conventional plasma-assisted CVD through decomposition of H ,S and Sil-I4 mixtures using :~ubstrate temperatures up to 500°C. When the deposition temperature is raised from 230 to 500°C. the concentration of S in the films decreases. The main effect of S alloying is a strong increase in IR mode at approximately 480 cm' which correspond.,+ to Si-S bond. The optical bandgap decreases dramatically with deposition temperature. The conductivity is found to be activated with the fermi level moving towards the conduction band edge up to S/St ratio of 0.51 * 10-" and as the S/St ratio further increases an increase in activation energy is observed which may be due to the increase in bandgap. The value of +r,, indicates conductioa in extended states, e~ 1998 Elsevier Science S.A.

The present study deliberates the optical properties of vacuum evaporated Pb 15 Se 85Àx Ge x (x = 0, 3, 6 at. %) thin films. The absence of sharp peaks in XRD hints about the non-crystalline nature. Transmission spectra (at normal incidence) are acquired in the spectral region of 400-2400 nm. A blue-shift in the transmission spectra hints about the increase in the band-gap. Experimentally, the band-gap is calculated using Tauc-extrapolation method. The band-gap transition is indirect and is found to increase with the substitution of Se by Ge in the glassy alloy. The optical density and penetration depth are calculated using the absorption coefficient data. The penetration depth increases with the increase in Ge-content and the cutoff wavelength shows a blue shift. The obtained optical properties are correlated with the physical parameters. Considering the topological model, it is revealed that the system shows a transition from floppy to the intermediate region. Lone pair electron value is greater than 3 and that favours the glass formation. The distribution of bonds in the present system is also examined using chemical bond approach (CBA) model and the system's cohesive energy (CE) is also calculated. The increase in cohesive energy shows that the average stabilisation energy of the system is increasing. This also reflects the increased value of experimentally calculated optical band-gap. The average heat of atomization also shows an increase with Ge addition. Conduction band potential seems to shift away from the Fermi level. The results are discussed based on the change in bond energy, bond length, total system energy and topological models applied on the system.

N-doping is often used to improve the photocatalytic properties of TiO 2 films in order to achieve visible light response. In this work, we study the effect of annealing treatment (temperature and atmosphere) on the structural and optical properties of undoped and N-doped TiO 2 films by means of X-ray diffraction (XRD), Atomic Force Microscopy (AFM), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE) and UV-VIS optical transmission spectroscopy. Porous five-layer TiO 2 films were deposited by sol-gel method on quartz substrate and thermally treated in oxygen or NH 3 flow at 500 and 600°C. Significant doping effect was achieved after annealing at 600°C in NH 3 and a shift in optical bandgap value down to visible range (2.69 eV) was observed.

Many novel applications are foreseen for the InN-containing materials especially when cheap, conductive substrates can be used. In this paper, we report on the growth of pure InN layers on germanium (Ge) (1 1 1) substrates. We found that high-quality InN can be grown on Ge(1 1 1) with plasma-assisted molecular beam epitaxy when using a thin GaN intermediate layer. On such intermediate GaN layers, 50 nm InN layers were grown and analyzed by RHEED, XRD, AFM, Hall and I-V measurements. Additionally, using ellipsometry we could determine that the optical bandgap of these InN layers lies around 0.85 eV. Our results indicate that Ge(1 1 1) is a promising substrate for vertical conducting devices, with In(Ga)N on top.

We report the two-photon absorption (TPA) spectra of poly(2 ,S-dibutoxy-p-phenylene acetylene) (PPA-OBO), poly(2,S-dioctyloxy-p-phenyiene vinylene) (PPV-DOO), and poly(3-hexylthiophene) (PATS) in the spectral range extending from 576 nm to 846 nm. Using the Z-scan technique on the polymers in solution , we measured a strong two-photon allowed transition in all three materials which we attribute to the mAg essential state. In the case of PPA-DBO and PPV-DOO , TPA peaks were coincident with dispersion in the nonlinear refractive indices as detected by reduced aperture Z scan. In all three polymers this peak occurs at approximately 1.3 the bandgap energy (Eg..,). The excitonic nature of the excited electronic states in PPA-DBO is indicated by the lack of a TPA band at or near the 1 Bu exciton position. Saturation was observed in the nonlinear index of refraction near spectral peaks. as well as an apparent reverse Kramers-Kronig effect.

The preparation of layers of amorphous Se by plasma-enhanced CVD using the hydride H2Se as precursor gas is described. Using a mixture of 15 vol.% H2Se in H,, partly crystallized films were obtained. Information wn&g the structure of the films was obtained fiom Raman spectroscopy. The spectra of amorphous Se i n d i d that the dominant molecular structure is the eight-membered ring andlor a chain with Se, molecular fiqpents. The optical transmission spectrum was recorded at different temperatures in the range 77-300 K. The optical bandgap E, was calculated fkom the optical absorption coefficients a using Tauc law : cdrv = C(hu-ETY, where hv is the photon energy. The temperature dependence of E, can be approximated by a linear relation : m~~-T,) =-7.6 10-4 x (T,-T,).

Optical and photoconductive properties of transparent SnO2 nanofibers, made from C22H44O4Sn via electrospinning and metallorganic decomposition, were investigated using Fourier transform infrared and ultraviolet (UV)/visible spectrometry and the two-probe method. Their optical bandgap was determined from their UV absorption edge to be 3.95–4.08eV. Their conductance responds strongly to UV light for a wavelength of 254nm: in air its steady-state on-to-off ratios are 1.31–1.56 (rise) and 1.25–1.33 (fall); its 90% rise and fall times are 76–96 and 71–111s, respectively. In a vacuum of about 10−4torr, its on-to-off ratios are higher than 35.6 (rise) and 3.4 (fall), respectively, and its 90% rise and fall times are longer than 3×104s.

This work investigates the growth of B-C-N layers by Chemical Vapor Deposition using methylamine borane (MeAB) as single-source precursor. MeAB has been synthesized and characterized, paying particular attention to the analysis of its thermolysis products, which are the gaseous precursors for B-C-N growth. Samples have been grown on Cu foils and transferred onto different substrates for their morphological, structural, chemical, electronic and optical characterizations. The results of these characterizations indicate a segregation of h-BN and Graphene-like (Gr) domains. However, there is an important presence of B and N interactions with C at the Gr borders, and of C interacting at the h-BN-edges, respectively, in the obtained nano-layers. In particular, there is significant presence of C-N bonds, at Gr/h-BN borders and in the form of N doping of Gr domains. The overall B:C:N contents in the layers is close to 1:3:1.5. A careful analysis of the optical bandgap determination of the o...

Oxysulfide systems undergo structural transformations upon illumination with laser light of near bandgap energy, as well as chalcogenide materials (glasses and films). In this paper, photoinduced effects such as photoexpansion and photobleaching were observed in GeS 2 þGa 2 O 3 (GGSO) films synthesized by electron beam evaporation. A surface expansion of the thin films and a shift to shorter wavelengths of the optical absorption edge were observed as a result of UV laser irradiation (wavelength of 351 nm) and they are dependent on laser power density, exposure time and film composition. These parameters were varied to evaluate and enhance the observed effects. In addition, the irradiated GGSO samples exhibited a decrease in refractive index, measured with a prism-coupling technique, which makes these films suitable candidates for applications as gratings and waveguides in integrated optics.

Several substantial physical and optical properties of non-crystalline Pb 10 Se 90-x Ge x bulk glasses and thin-films (0.0 ≤ x ≤ 10 at.%) have been studied. Bulk and film samples were synthesized via melt-quench and thermalevaporation processes, respectively. The films were fabricated on pre-cleaned glass substrates under vacuum ≈5 × 10 −4 Pa. The non-crystallinity of studied samples has investigated by X-ray diffractograms. A goodmatching between selected compositional elements and those measured experimentally was confirmed by energy-dispersive X-ray analysis. The absorption nature of samples was studied in detail, where absorbance, absorption coefficient, skin depth and skin effect were discussed. The indirect transition bandgap energy was studied and its value decreases as Ge-content increases. Cohesive energy, average coordinate number, average atomization heat, lone-pair electrons, mean-bond energy, and others were discussed. Based on density values, many parameters were estimated and studied, like compactness, packing density, atomic densities, polaron radius, field strength. All studied characterizations are strongly dependent upon Ge-percentages.

The optical and structural properties of Ge 20 Se 80 , Ge 25 Se 75 and Ge 30 Se 70 bulk glasses and Ag x (Ge 0.20 Se 0.80) 100Àx thin films, where x = 0, 6, 11, 16, 20 and 23 at.% were studied. All samples were confirmed as amorphous according to XRD. The Raman spectra showed increase in 260 cm À1 and 237 cm À1 and decrease in 198 cm À1 and 216 cm À1 bands with different Se content in the bulk samples. The optical bandgap energy of bulk samples decreased (2.17-2.08 eV) and refractive index increased (2.389-2.426 at 1550 nm) with increasing Se content in bulk glasses. The Ge 20 Se 80 thin films were prepared by vacuum thermal evaporation from Ge 30 Se 70 bulk glasses. The Raman spectra of the films showed that peaks at 260 cm À1 and 216 cm À1 decreased their intensities with increasing Ag content in the thin films. The significant red shift of bandgap energy occurred upon different Ag content. The optically induced dissolution and diffusion resulted in graded refractive index profile along the film thickness caused by different Ag concentration. The refractive index increased from the substrate side to the top of thin films. The graded profile was getting more uniform with increasing content of silver in the thin film.

In this work we report on pulsed laser deposition of gallium-doped zinc oxide (ZnO:Ga) transparent-cooducting thin films grown on glass at different subslrate temperatures. A widening in !he optical bandgap and a good gallium-doping efficiency were observed in the films when the substrate temperature was raised from 150 °C to 300 °C, as determined from optical and electrical measurements. X-ray diffraction measurements revealed that the films grow preferentially oriented in the [002] crystallographic direction of the ZnO grains. The crystallini .ty of the films was also found to be strongly dependent on the substratc deposition temperature. Tha ZnO:Ga transparent films had excellent transmittance (85%) in the visible spectrum and a low electrical resistivity value (7 × 10-4 ,O cm) in 200 nm thickness samples deposited on glass by laser ablation at 300 *C.

The optical and structure properties of Cadmium oxide (CdO) thin film prepared by electrochemical deposition method at different times (15, 30 and 60) min were investigated in this paper. Results of optical Transmission, absorption, reflection spectra, optical conductance, refractive index, extension coefficient, real and imaginary dielectric constants studies are reported. The optical transmittance of the CdO thin film which formed at room temperature was 20% at wavelength ≈350 nm then increases to 60% at wavelength ≈1100 nm for thin film of CdO. The band-gap was also calculated from the equation relating absorption coefficient with the wavelength. The energy band gap changes from 2.3eV (Bulk CdO) to 2.45eV (CdO thin film). The plotted graphs show the optical characteristics of the film which varied with the wavelength and the photon energy. The optical conductance and band-gap indicated that the film is transmitting within the visible range. The dielectric constant and optical con...

TiO 2 and TM-doped TiO 2 (TM = Re, Ru) anatase crystalline nanopowders were synthesized by a simple hydrothermal method. Samples with nominal TM/Ti ratio of 0.01 were prepared for this study. Their structural, microstructural and optical properties were studied. The lattice parameters of the different prepared samples were calculated and their mean crystallite sizes determined to be in the range 15-7 nm, the lower values being obtained for the doped crystallites. The samples' specific surface areas were determined and correlated with their mean crystallite sizes. The incorporation of the dopant elements results in an increase of the optical absorption in the visible range. The samples' optical bandgap and Urbach energies were calculated from UV-Vis diffuse reflectance spectra. The photocatalytic behavior of the synthesized samples was investigated for the rhodamine B and phenol degradation processes, the kinetics of the different photo-oxidation reactions being also studied. The results showed that doping either with Re or Ru can lead to enhancement of the TiO 2 photocatalytic efficiency and that, among the synthesized samples Ru-doped TiO 2 is the most efficient photocatalyst for both the Rhodamine B and phenol photodegradation reactions.

The effect of fluorine incorporation on the thermal stability of hydrogenated amorphous carbon (a-C:H) films was investigated by Raman spectroscopy, infrared (IR) absorption, X-ray photoelectron spectroscopy (XPS), as well as measurements of the film thickness. Results of IR and XPS analysis suggest that the total fluorine content after annealing corresponds to the amount of fluorine bonded to carbon in the as-deposited films. The evolution of Raman spectra obtained on films before annealing indicates that progressive fluorine incorporation is accompanied by graphitisation. In particular, the decrease in the optical bandgap is interpreted as a clustering of the existing sp carbon sites. Under thermal annealing, the film structures are very stable and the 2 films do not show any change in refractive index values with respect to the original as-deposited films, for all fluorine contents. This behaviour is most likely due to extended graphitisation, with no sp clustering. The experimental findings suggest that the 2 strength of the CF bonding configuration promotes thermal stability of the films.

Processible nanocomposites of carboxyl functionalized conducting polymer "polyanthranilic acid" (PANA) with multiwalled carbon nanotubes (MWNTs) are prepared using two different synthesis routes viz. single phase and two phase polymerization. The novel nanocomposite materials are characterized using X-ray diffraction (XRD), thermogravimetric (TGA), electrochemical impedance (EI), scanning electron microscope (SEM) and high resolution transmission electron microscope (HRTEM) techniques for their structural, thermal, electrochemical and surface morphological properties. SEM and HRTEM images are confirmed that nanotubes are dispersed uniformly in polymer matrix and polymer chains wrap around the nanotubes walls. The interaction between MWNTs and PANA is analyzed by Raman and Fourier transform infrared (FTIR) spectroscopy. UV-vis spectroscopic technique is used to obtain the optical bandgap of nanocomposites. PANA-MWNTs nanocomposites are used for the first time for fabrication of sandwich type devices with a configuration of metal Al/PANA-MWNTs nanocomposite/indium tin oxide coated glass (ITO). The current density-voltage (J-V) and capacitance-voltage (C-V) characteristics of the Schottky diode are subsequently used for extracting electronic parameters of the devices. These measurements revealed that the junction electrical parameters depend strongly on the synthesis route for preparation of nanocomposites.

Nitrogen ions with different doses were implanted into titanium nitride (TiN) thin films coated on mild steel substrate at 100 keV with doses of 5610 15 and 5610 16 ions/cm 22. The samples were characterised by X-ray diffraction and found to have fcc structure with preferential orientation along the (200) plane. The presence of nitride (TiN), oxynitride (TiO x N y) and oxide (TiO 2) were identified on the surface of the film as observed from X-ray photoelectron spectroscopy analysis. Scanning electron microscopy image analysis of implanted samples showed changes in the surface after ion implantation. Energy dispersive X-ray spectroscopy was used to determine the chemical composition of as prepared and ion implanted samples. The surface topography was studied by atomic force microscopy. Characteristic peaks were observed in the 150-300 and 400-600 cm 21 ranges from laser Raman studies.

Four low bandgap polymers, combining an alkyl thiophene donor with benzo[c][1,2,5]thiadiazole, 2,3diphenylquinoxaline, 2,3-diphenylthieno[3,4-b]pyrazine and 6,7-diphenyl-[1,2,5]thiadiazolo[3,4-g] quinoxaline acceptors in a donor-acceptor-donor architecture, were synthesized via FeCl 3 oxidative polymerization. The molecular weights of the polymers were improved by introducing o-dichlorobenzene (ODCB) as the reaction solvent instead of the commonly used solvent, chloroform. The photophysical, electrochemical and photovoltaic properties of the resulting polymers were investigated and compared. The optical bandgaps of the polymers vary between 1.0 and 1.9 eV, which is promising for solar cells. The devices spin-coated from an ODCB solution of P1DB:[70]PCBM showed a power conversion efficiency of 1.08% with an open-circuit voltage of 0.91 V and a short-circuit current density of 3.36 mA cm À 2 under irradiation from an AM1.5G solar simulator (100 mW cm À 2).

The present study deliberates the optical properties of vacuum evaporated Pb 15 Se 85Àx Ge x (x = 0, 3, 6 at. %) thin films. The absence of sharp peaks in XRD hints about the non-crystalline nature. Transmission spectra (at normal incidence) are acquired in the spectral region of 400-2400 nm. A blue-shift in the transmission spectra hints about the increase in the band-gap. Experimentally, the band-gap is calculated using Tauc-extrapolation method. The band-gap transition is indirect and is found to increase with the substitution of Se by Ge in the glassy alloy. The optical density and penetration depth are calculated using the absorption coefficient data. The penetration depth increases with the increase in Ge-content and the cutoff wavelength shows a blue shift. The obtained optical properties are correlated with the physical parameters. Considering the topological model, it is revealed that the system shows a transition from floppy to the intermediate region. Lone pair electron value is greater than 3 and that favours the glass formation. The distribution of bonds in the present system is also examined using chemical bond approach (CBA) model and the system's cohesive energy (CE) is also calculated. The increase in cohesive energy shows that the average stabilisation energy of the system is increasing. This also reflects the increased value of experimentally calculated optical band-gap. The average heat of atomization also shows an increase with Ge addition. Conduction band potential seems to shift away from the Fermi level. The results are discussed based on the change in bond energy, bond length, total system energy and topological models applied on the system.

Cadmium oxide (CdO) thin films were electrodeposited on indium doped tin oxide glass substrates from aqueous solution and subsequently annealed at higher temperatures up to 600 • C in air, oxygen and inert gas ambients, respectively. The effect of annealing on the morphology, crystalline quality and optical properties of CdO films was investigated. The CdO films were polycrystalline in nature and the grain size strongly depended on the annealing temperature as well as annealing ambient. CdO films were highly transparent in the visible region of the spectrum. The transmittance of the films decreased with increasing annealing temperatures in all the three ambients. Optical bandgap of the nanocrystalline CdO films varied in the range of 2.20-2.54 eV.

Optical and photoconductive properties of transparent SnO2 nanofibers, made from C22H44O4Sn via electrospinning and metallorganic decomposition, were investigated using Fourier transform infrared and ultraviolet (UV)/visible spectrometry and the two-probe method. Their optical bandgap was determined from their UV absorption edge to be 3.95–4.08eV. Their conductance responds strongly to UV light for a wavelength of 254nm: in air its steady-state on-to-off ratios are 1.31–1.56 (rise) and 1.25–1.33 (fall); its 90% rise and fall times are 76–96 and 71–111s, respectively. In a vacuum of about 10−4torr, its on-to-off ratios are higher than 35.6 (rise) and 3.4 (fall), respectively, and its 90% rise and fall times are longer than 3×104s.

Processible nanocomposites of carboxyl functionalized conducting polymer "polyanthranilic acid" (PANA) with multiwalled carbon nanotubes (MWNTs) are prepared using two different synthesis routes viz. single phase and two phase polymerization. The novel nanocomposite materials are characterized using X-ray diffraction (XRD), thermogravimetric (TGA), electrochemical impedance (EI), scanning electron microscope (SEM) and high resolution transmission electron microscope (HRTEM) techniques for their structural, thermal, electrochemical and surface morphological properties. SEM and HRTEM images are confirmed that nanotubes are dispersed uniformly in polymer matrix and polymer chains wrap around the nanotubes walls. The interaction between MWNTs and PANA is analyzed by Raman and Fourier transform infrared (FTIR) spectroscopy. UV-vis spectroscopic technique is used to obtain the optical bandgap of nanocomposites. PANA-MWNTs nanocomposites are used for the first time for fabrication of sandwich type devices with a configuration of metal Al/PANA-MWNTs nanocomposite/indium tin oxide coated glass (ITO). The current density-voltage (J-V) and capacitance-voltage (C-V) characteristics of the Schottky diode are subsequently used for extracting electronic parameters of the devices. These measurements revealed that the junction electrical parameters depend strongly on the synthesis route for preparation of nanocomposites.

The optical parameters are critical factors for the development of optoelectronic devices. Here the authors report the study of optical parameters based on some empirical relationship for Ga doped Ge-Te-Se. These relationships are based on the specific models proposed for the calculation of binary, ternary, and quaternary chalcogenide glasses for the specific range of energy gap. The empirically calculated values of optical parameters from these models are compared with the experimental value of the chalcogenide thin films. The values of refractive index have been calculated from the optical bandgap. Some relations proposed by different authors show good agreement between the experimental and calculated values. It has been concluded that Ravindra & Gupta, Reddy & Duffy, Kumar & Singh, and Reddy provide the satisfactory results.

hand, This balance generally results in a temperature in the town that is slightly higher than in the surrounding country. Using solar energy saves imported fuels on one hand, but increases the absorption of solar radiation on the other hand. Simple, steady state models are used to assess the channe of heat released to the environment when reolacine the use of clas&al fuels by solar powered plants, on both theglob; and city scale. The conclusion is that, in most cases, this will reduce the heat released to the environment. The exception is cooling, for which a good solar alternative does not exist today.

Zn-Se bilayer thin films have been deposited on well cleaned glass substrate by using the vacuum evaporation technique under a vacuum of 5 × 10-5 Torr. These bilayer thin films were modified by ion irradiation, vacuum annealing and rapid thermal annealing. X-ray diffraction and optical measurement of as deposited and modified samples have been performed. The formation of cubic ZnSe phase has been observed in modified samples and optical band gap value has been found to be in the range of ZnSe compound.

Spectroellipsometrical (SE) investigations in 1.5-4.0 eV spectral range were done for optical characterization of porous silicon (PS) thin films. A mixing of a crystalline silicon and amorphous silicon with voids using Effective Medium Approximation [D.E. Aspnes, Phys. Rev. B 27 (1983) 985] is used to calculated the refraction index (n) and the absorption constant (k) of the PS layers. Measurements were done on PS obtained electrochemically on p ϩ silicon substrates. From the dispersion spectra of the refractive index, using the Wemple DiDomenico model [S.H. Wemple, M. DiDomenico Jr., Phys. Rev. B 3 (1971) 41338], the values for optical band gap, energy oscillator and dispersion energy for PS (model's parameters introduced by Wemple) are evaluated.

Thin films of arsenic triselenide doped with silver were prepared by thermal evaporation of the bulk-quenched materials. Structural investigations have been carried out by using X-ray diffraction and X-ray fluorescence. The differential scanning calorimetry is used to measure the glass transition temperature of the samples under investigation. The Swanepoel method based on the use of the maxima and minima in the interference fringes has been successfully utilized for accurate determination of the film thickness and optical parameters. The technique used took into consideration the non-uniform nature in the film thickness. Using the exact calculation of the film thickness the real refractive index, the extinction coefficient, the optical bandgap energy and the oscillator and dispersion energies were obtained as a function of the silver contents. The DC conductivity was found to depend on the silver content.

Zinc (Zn)-doped nickel oxide (NiO) thin films were fabricated using spin coating technique for humidity sensing applications. Nickel acetate tetrahydrate was used as a precursor material and zinc acetate was used as the dopant source. The films were annealed at various annealing temperatures of 400 ºC, 500 ºC and 550 ºC in ambient. X-ray diffraction (XRD) was used to investigate the structural properties of the samples, while ultravioletvisible spectroscopy (UV-Vis) was used to characterize the optical properties of the samples. The thickness of the samples was measured using a surface profiler which the thickness are 178.8 nm, 148.0 nm, and 116.9 nm for samples annealed at 400 ºC, 500 ºC and 550 º, respectively. The humidity sensing properties for each sample were characterized using humidity chamber equipped with humidity sensing measurement system which the relative humidity (RH) value was set between 40% RH and 90% RH. The XRD results showed that all samples were in the amorphous state. The average transmittance in the visible region obtained from the UV-vis spectra exceeded 90% transmission for all samples. The highest bandgap obtained is 3.6 eV for Zn-doped NiO annealed at 500 ºC. The humidity sensing results show that a high sensitivity sensor could be produced using sample annealed at 550 ºC.

Beryllium nitride thin films, which are candidates for optoelectronic applications, have been grown by pulsed laser deposition on silicon substrates. The films were prepared by ablating a beryllium foil in N environment at several pressures and substrate 2 temperatures. Real-time ellipsometric monitoring for the period of deposition were carried out by a multiwavelength ellipsometer in the 1.625(hn(4.405 eV photon-energy range. After its completion, the films were characterized in situ by electron spectroscopies and ex situ by atomic force and scanning electron microcopies. A model for the growth of beryllium nitride was applied to reproduce the optical measurement and concurrently, the refractive index from the visible to the near ultraviolet spectral region was calculated. The estimated optical bandgap correlates closely with previously published theoretical results.

Electronic and optical properties of AlÀGa codoped ZnO thin films were investigated by post-annealing. The lowest resistivity of the AleGa codoped ZnO films was observed from the 450 C-annealed sample. The Fermi-level shift of the AlÀGa codoped ZnO film was w0.6 eV from x-ray photoelectron spectroscopy, and the widening of optical-bandgap in the AlÀGa codoped ZnO film was w0.3 eV. The correlations of optical-bandgap with Fermi-level shift and conduction band filling were suggested by schematic band diagrams.

We present degenerate and nondegenerate two-photon absorption spectra in a series of CdSe and CdTe quantum dots. The measurements show that the two-photon absorption (2PA) spectrum is strongly dependent on the quantum dot size and that the 2PA coefficient decreases as the quantum dot size decreases, and it is larger for the frequency nondegenerate process. Previously we had shown a theoretical analysis of these results based on a simple model using the effective mass approximation. Although this model works well for larger quantum dots, it fails for the smaller ones. Here we use the more realistic p k r r • model for the band structure and consider the hole band mixing in quantum dots to describe our data. This theory better describes the spectral structures for smaller quantum dots and also predicts the decrease of the 2PA coefficient with the decrease of quantum dot size. This is due to the reduction of the number of possible transitions and the blue shift of the optical bandgap from quantum confinement. This theory predicts the reduction of the 2PA coefficient with size, although our experimental results show an even stronger reduction.