Reduction In Optical Band Gap Research Articles

A tunable amorphous p-type ternary oxide system: The highly mismatched alloy of copper tin oxide

The approach of combining two mismatched materials to form an amorphous alloy was used to synthesise ternary oxides of CuO and SnO2. These materials were analysed across a range of compositions, and the electronic structure was modelled using density functional theory. In contrast to the gradual reduction in optical band gap, the films show a sharp reduction in both transparency and electrical resistivity with copper contents greater than 50%. Simulations indicate that this change is caused by a transition from a dominant Sn 5s to Cu 3d contribution to the upper valence band. A corresponding decrease in energetic disorder results in increased charge percolation pathways: a “compositional mobility edge.” Contributions from Cu(II) sub band-gap states are responsible for the reduction in optical transparency.

Optical properties of electron irradiated Bombyx mori silk fibroin films

Energetic electron irradiation effects on the optical properties of Bombyx mori silk fibroin (SF) films are studied using UV–visible spectrophotometer. The recorded UV–visible absorption and transmission graphs have been used to determine the optical band gap (E g ), refractive index (n), extinction coefficient (k), optical conductivity (σ opt ) and dielectric constants (e *) of the films. The studies have shown a reduction in optical band gap and increase in refractive index with increasing electron dosage. It is also observed that dielectric constants increase with increasing photon energy. The observed optical changes have been tried to be correlated with the structural changes, revealed through FT-IR spectroscopy. These results indicate that the refractive index of SF films can be changed efficiently by electron irradiation.

Influence of 400 keV carbon ion implantation on structural, optical and electrical properties of PMMA

Ion implantation is a useful technique to modify surface properties of polymers without altering their bulk properties. The objective of this work is to explore the 400keV C+ ion implantation effects on PMMA at different fluences ranging from 5×1013 to 5×1015ions/cm2. The surface topographical examination of irradiated samples has been performed using Atomic Force Microscope (AFM). The structural and chemical modifications in implanted PMMA are examined by Raman and Fourier Infrared Spectroscopy (FTIR) respectively. The effects of carbon ion implantation on optical properties of PMMA are investigated by UV–Visible spectroscopy. The modifications in electrical conductivity have been measured using a four point probe technique. AFM images reveal a decrease in surface roughness of PMMA with an increase in ion fluence from 5×1014 to 5×1015ions/cm2. The existence of amorphization and sp2-carbon clusterization has been confirmed by Raman and FTIR spectroscopic analysis. The UV–Visible data shows a prominent red shift in absorption edge as a function of ion fluence. This shift displays a continuous reduction in optical band gap (from 3.13 to 0.66eV) due to formation of carbon clusters. Moreover, size of carbon clusters and photoconductivity are found to increase with increasing ion fluence. The ion-induced carbonaceous clusters are believed to be responsible for an increase in electrical conductivity of PMMA from (2.14±0.06)×10−10 (Ω-cm)−1 (pristine) to (0.32±0.01)×10−5 (Ω-cm)−1 (irradiated sample).

Mechanosynthesis, deposition and characterization of CZTS and CZTSe materials for solar cell applications

Mechanosynthesis of nanocrystalline powders of CZTS and CZTSe by ball milling technique and the physical properties of thermally evaporated CZTS and CZTSe thin films as a function of substrate temperature are investigated. Nanocrystalline Cu–Zn–Tin–Sulphide (CZTS) and Cu–Zn–Tin–Selenide (CZTSe) powders synthesized by ball milling at different milling time using the source materials of Cu, Zn, Sn, S (or) Se in the ratio 2:1:1:4 are investigated. The above synthesized powder was thermally evaporated on glass substrate kept at room temperature and 673K under a vacuum of 10−6mbar to prepare quaternary compound semiconducting thin films in a single step process. The synthesized powder and deposited CZTS and CZTSe thin films belong to tetragonal crystal system. Raman spectra reveal that the synthesized nanocrystals are pure without any secondary phases. A gradual reduction in optical bandgap of films was observed with increasing substrate temperature due to increased crystallinity of the films. The changes in surface morphology of the films with respect to substrate temperature were studied by scanning electron microscopy and atomic force microscopy. Electrical studies indicate that the deposited films have p-type conductivity.

MnS thin films prepared by a simple and novel nebulizer technique: report on the structural, optical, and dispersion energy parameters

The present work describes different properties of manganese sulfide (MnS) thin films deposited on glass substrate at 300 °C by a simple and novel nebulized spray technique. The annealing temperature induced enhancement in polycrystalline nature of films was observed from the X-ray diffraction study. The maximum optical transmittance of 98 % was observed for the as-deposited thin film. The observed transmittance threshold shift towards higher wavelength region (red shift), indicated a systematic reduction in optical band gap (E g ; 3.95–3.33 eV) of the films with increasing annealing temperature. The increase in Urbach energy with annealing temperature is due to the presence of localized states, which is consistent with the E g data. The optical constants, dielectric parameters, porosity, dispersion energy parameters, relaxation time, and optical non-linear susceptibility of the films were also evaluated from the optical transmittance data. These results are discussed and correlated well in the light of possible mechanisms underlying the phenomena. The Raman spectra show two peaks at 355 and 637 cm−1 which correspond to the vibrations of Mn–S bonds. The strong band edge emission observed in the PL spectra indicates the high optical quality of deposited MnS thin films. The paramagnetic behavior of the film was confirmed from the M–H plot.

“Darker-than-Black” PbS Quantum Dots: Enhancing Optical Absorption of Colloidal Semiconductor Nanocrystals via Short Conjugated Ligands

Colloidal quantum dots (QDs) stand among the most attractive light-harvesting materials to be exploited for solution-processed optoelectronic applications. To this aim, quantitative replacement of the bulky electrically insulating ligands at the QD surface coming from the synthetic procedure is mandatory. Here we present a conceptually novel approach to design light-harvesting nanomaterials demonstrating that QD surface modification with suitable short conjugated organic molecules permits us to drastically enhance light absorption of QDs, while preserving good long-term colloidal stability. Indeed, rational design of the pendant and anchoring moieties, which constitute the replacing ligand framework leads to a broadband increase of the optical absorbance larger than 300% for colloidal PbS QDs also at high energies (>3.1 eV), which could not be predicted by using formalisms derived from effective medium theory. We attribute such a drastic absorbance increase to ground-state ligand/QD orbital mixing, as inferred by density functional theory calculations; in addition, our findings suggest that the optical band gap reduction commonly observed for PbS QD solids treated with thiol-terminating ligands can be prevalently ascribed to 3p orbitals localized on anchoring sulfur atoms, which mix with the highest occupied states of the QDs. More broadly, we provide evidence that organic ligands and inorganic cores are inherently electronically coupled materials thus yielding peculiar chemical species (the colloidal QDs themselves), which display arising (opto)electronic properties that cannot be merely described as the sum of those of the ligand and core components.

Evolution of Raman Scattering and Electronic Structure of Ultrathin Molybdenum Disulfide by Oxygen Chemisorption

Molybdenum disulfide (MoS2) has attracted vast interest in the fields of electronic, optoelectronic, and valleytronic applications due to its unique properties. Because of its low dimensionality, MoS2 is susceptible to oxygen absorption in ambient environments, which can alter its electrical and optical properties. Here, a new method to introduce oxygen chemisorption in ultrathin MoS2 by controlled oxygen plasma treatment is presented. Using Raman spectroscopy, a red shift in the frequency of E12g mode with increasing oxygen chemisorption is found, whereas, the frequency of A1g mode is fixed. Interestingly, the absorption peak in the photoluminescence spectra red shifts, indicating an optical band gap reduction upon oxygen chemisorption. The behaviors of these different shifts are reproduced and elucidated by density functional theory calculations. It is found that the red shift of the E12g Raman peak is caused by a softening of in‐plane Mo‐S force constants, while the red shift in the photoluminescence peak is due to a reduction of the electronic band gap in oxygen‐chemisorbed MoS2. The results shine light on the fundamental understanding of chemical interactions between oxygen and MoS2 and provide an alternative way to achieve band gap engineering in MoS2.

Tuning the Refractive Index and Optical Band Gap of Silk Fibroin Films by Electron Irradiation

TheBombyx morisilk fibroin (SF) films were prepared by solution casting method and effects of electron beam on the optical properties and optical constants of the films have been studied by using UV-Visible spectrophotometer. Optical properties like optical band gapEg, refractive indexn, extinction coefficientk, optical conductivityσopt, and dielectric constantsε∗of virgin and electron irradiated films were determined by using UV-Visible absorption and transmission spectra. It was found that the reduction in optical band gap and increase in refractive index with increasing radiation dosage was observed. It is also observed from results that there is increase in dielectric constants with increasing photon energy. The observed optical changes have been tried to be correlated with the structural changes, revealed through FT-IR spectroscopy. The present study is quite important for tailoring the optical responses of SF films as per specific requirements.

Morphological, structural and optical properties of silver treated zinc oxide thin film

Zinc oxide (ZnO) thin film with an ultrathin layer of silver (Ag) atop was fabricated on glass substrate and a simple post-deposition annealing was carried out. A systematic observation was conducted to monitor morphological, structural and optical properties. The detailed morphological analysis by scanning electron microscope revealed that ZnO film surface became irregular with small pores dispersed all over the surface after the treatment, whereas the surface became smother with an ultrathin layer of Ag atop. Nucleation of Ag nanoparticles was observed initiated at the beginning of treatment. Preferential film orientation along c-axis with wurtzite hexagonal structure and a peak shift to red after treatment was confirmed by X-ray diffraction. Further investigation revealed that a stress relaxation from −0.51 GPa (compressive stress) to 0.58 GPa (tensile stress), grain growth from 13 to 31 nm and recrystallization with decreasing lattice constant (c) from 5.2655 to 5.1395 A occurred after treatment. A reduction in optical band gap from 3.31 to 3.25 ± 0.01 eV was observed after the treatment with a slight increase in transmittance. At higher annealing temperatures, the weak photoluminescence emission did show blue shift from 367 to 364 nm and 362 nm. Elemental and compositional analysis was carried out by X-ray photoelectron spectroscopy. Such characteristics shift in structural and optical properties, in fact, plays a crucial role in determining device performances and overall efficiency.

Composition and thickness dependant optical study of a-Pb–Se–Ge–Sn glassy thin films

Thickness and compositional dependence of optical properties of Pb9Se71Ge20−xSnx (8 ≤ x ≤ 12) glass has been studied. Various optical constants such as refractive index, extinction coefficient and optical band gap have been determined by analyzing optical transmittance data in the wavelength range of 200–3,500 nm. Density of localized states and disorder plays a crucial role in deciding the optical properties of amorphous semiconductors. Refractive index and extinction coefficient increase as Sn content increase in material. With the rise in thickness, there may be increase in order of short range order of the film and continuous random network simultaneously, result in reduced band gap. The isoelectronic substitutions of Ge by Sn in the glassy system also contribute to reduction in optical band gap of the material.

Structural and optical study of Li doped CuO thin films on Si (100) substrate deposited by pulsed laser deposition

• Cu 1 − x Li x O ( x = 0.0, 0.05, 0.07 and 0.09) thin films are successfully grown by PLD. • Raman and PL spectra confirm the single-phase formation of CuO thin films. • FESEM and AFM show that the grain size of Cu 1 − x Li x O thin films is ∼35 to 79 nm. • A reduction in optical band gap is observed from 2.99 to 2.76 eV for x = 0 to 0.09. In the present study, we report structural and optical properties of Cu 1 − x Li x O thin films grown on Si (1 0 0) substrates by pulsed laser deposition at different doping concentrations ( x = 0.0, 0.05, 0.07 and 0.09). The XRD spectra indicates the formation of polycrystalline CuO thin films and the crystallite size, calculated through XRD data is found to be increased from 7 to 14 nm for the samples with x = 0 to 0.09, respectively. FESEM analysis shows that the average size of Cu 1 − x Li x O nanostructures increases from 47 to 97 nm and AFM analysis also shows that the average size of Cu 1 − x Li x O nanostructures increases from 35 to 79 nm by increasing the Li doping concentrations from x = 0 to 0.09, respectively. Energy dispersive X-ray analysis with elemental mapping of the Li-doped CuO thin films shows that the Li dopants are incorporated homogeneously into the CuO thin film matrix. Moreover, Raman and photoluminescence spectra also confirm the single-phase formation of CuO thin films. A significant reduction in optical energy band gap is observed from 2.99 to 2.76 eV with an increase of Li concentration from 0 to 9%, respectively.

Structural, Optical, and Ferroelectric Behaviors of Cu1−x Li x O (0 ≤ x ≤ 0.09) Nanostructures

We report structural, optical, and ferroelectric behaviors of lithium-doped copper oxide (Cu1−x Li x O with x = 0.0, 0.05, 0.07, and 0.09) nanostructures synthesized by hydrothermal method. The XRD pattern indicates the pure phase formation of CuO without any impurity, and the crystallite size is found to be increases for x = 0–0.07 and decreases for x = 0.09. FESEM analysis shows that the average size of Cu1−x Li x O nanostructures increases with the increasing the Li-doping concentrations up to 7% and then decreases for 9% Li doping concentration. Moreover, Raman and photoluminescence spectrum also confirm the phase formation of CuO. A significant reduction in optical band gap is observed up to x = 0.07, and then band gap increases for x = 0.09 due to segregation of the impurities on the surface or grain boundaries, which may suppress the grain growth and results the enhancement in optical band gap. Moreover, a weak ferroelectricity is observed in CuO nanostructures for pure and 9% Li doping through polarization versus electric field (P–E).

Synergic effect of the TiO2-CeO2 nanoconjugate system on the band-gap for visible light photocatalysis

Abstract The TiO 2 -CeO 2 photocatalytic system in films is proposed here, in order to obtain photocatalytic systems that can be excited by solar light. The films were obtained through the electrophoretic deposition (EPD) of TiO 2 -CeO 2 gel on sputtered Ti Corning glass substrates. The synergic effect of CeO 2 in TiO 2 films was analyzed as a function of the optical band gap reduction at different concentrations (1, 5, 10, and 15 mol%). The effect of two thermal treatments was also evaluated. The lowest band gap value was obtained for the sample with 5 mol% ceria that was thermally treated at 700 °C. The nanostructured films were characterized by Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high angle annular dark field (HAADF), high resolution transmission electron microscopy (HRTEM), and atomic force microscopy (AFM). The nanocomposites were formed by TiO 2 and CeO 2 nanoparticles in the anatase and fluorite type phases, respectively.

Irradiation effects of 65 MeV Kr-ions on structure and optical band-gap of nc-Si:H films

In this paper, hydrogenated nano-crystalline silicon (nc-Si:H) films with thickness of 1.0μm were irradiated with 65MeV Kr-ions, and the irradiation induced modification on structural and optical properties of the films are investigated. Characterization of the samples with X-ray diffraction shows that the average crystallite size decreases slightly from 10.9 to 9.4nm, and the integrated intensity ratios of (220) to (111) peak and (311) to (111) peak remain nearly constant after irradiation. The Raman spectra results reveal that the crystalline fraction of the films decreases considerably from 65.6% to 10.3% and at same time the bond angle deviation of amorphous phase increases obviously from 7.7° to 11.5° with increasing the ion fluence. Moreover, the UV–Vis transmittance spectra results show that the optical band-gap decreases from 2.14 to 1.55eV after irradiation. The irradiation induced atomic displacements leads to the amorphization of crystalline phase and the reduction in structural order of amorphous phase in the nc-Si:H films. The improvement of light absorption and the broadening of valence and conduction band-tails, which related to the decrease of crystalline fraction and short range order, respectively, are responsible for the reduction of optical band-gap.

Modification of structure and optical band-gap of nc-Si:H films with 30 and 196 MeV Kr-ions

In the present work, experimental results on irradiation effects of 30 and 196MeV Kr-ions on nc-Si:H films are presented. The irradiation fluences are 5.0×1013, 1.0×1014 and 2.0×1014Kr-ions/cm2, respectively. Irradiation induced modification on microstructure and optical properties of nc-Si:H films are studied by X-ray diffraction, Raman scattering and UV–Vis-NIR spectroscopy. The analyses results indicate that the irradiations lead to the decrease of crystalline fraction and crystallite size, as well as the increase of bond angle variation of amorphous network. The structural changes become more obvious with the nuclear energy loss increases. Moreover, with the ion fluence increases, the optical band-gap for samples irradiated with 30MeV Kr-ions decreases gradually, while it for samples irradiated with 196MeV Kr-ions decreases significantly at first and then remains nearly constant. It is considered that the irradiation induced atomic displacements in the films lead to the amorphization of crystalline phase and the decrease of short-range order in amorphous phase, and then result in the reduction of optical band-gap.

Structural refinement, growth mechanism, infrared/Raman spectroscopies and photoluminescence properties of PbMoO4 crystals

Lead molybdate (PbMoO4) crystals were synthesized by the co-precipitation method at room temperature and then processed in a conventional hydrothermal (CH) system at low temperature (70°C for different times). These crystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, micro-Raman (MR) and Fourier transformed infrared (FT-IR) spectroscopies. Field emission scanning electron microscopy images were employed to observe the shape and monitor the crystal growth process. The optical properties were investigated by ultraviolet–visible (UV–Vis) absorption and photoluminescence (PL) measurements. XRD patterns and MR spectra indicate that these crystals have a scheelite-type tetragonal structure. Rietveld refinement data possibilities the evaluation of distortions in the tetrahedral [MoO4] clusters. MR and FT-IR spectra exhibited a high mode ν1(Ag) ascribed to symmetric stretching vibrations as well as a large absorption band with two modes ν3(Eu and Au) related to anti-symmetric stretching vibrations in [MoO4] clusters. Growth mechanisms were proposed to explain the stages involved for the formation of octahedron-like PbMoO4 crystals. UV–Vis absorption spectra indicate a reduction in optical band gap with an increase in the CH processing time. PL properties of PbMoO4 crystals have been elucidated using a model based on distortions of tetrahedral [MoO4] clusters due to medium-range intrinsic defects and intermediary energy levels (deep and shallow holes) within the band gap.

Refractive Index Tailoring of Poly(methylmethacrylate) Thin Films by Embedding Silver Nanoparticles

Nanocomposite films of Poly (methylmethacrylate) with different concentration of silver nanoparticles were prepared by ex-situ method. Firstly, silver nanoparticles were obtained by reducing the aqueous solution of silver nitrate with sodium borohydride then Ag-PMMA films were prepared by mixing colloidal solution of silver nanoparticles with solution of polymer. Thin solid films were structurally characterized using UV-VIS spectroscopy and TEM. The appearance of surface plasmon resonance peak, characteristic of silver nanoparticles at 420 nm in UV-VIS absorption spectra of Ag-PMMA films confirms the formation of Ag-PMMA nanocomposite. TEM showed Ag nanoparticles of average size 8 nm embedded in PMMA matrix. Analysis of absorption and reflection data indicates towards the reduction in optical band gap and increase in refractive index of the resulting nanocomposite. The synthesized Ag-PMMA nanocomposite has been found to be more conducting than PMMA as ascertained using I-V studies. The decrease in band gap and increase in conductivity can be correlated due to the formation of localized electronic states in PMMA matrix due to insertion of Ag nanoparticles. The PMMA thin films with dispersed silver nanoparticles may be useful for nanophotonic devices.

Investigation on structural, optical, morphological and electrical properties of thermally deposited lead selenide (PbSe) nanocrystalline thin films

In this paper, we report the substrate temperature induced changes in physical properties of thermal evaporated lead selenide (PbSe) thin films from the chemically synthesized nanocrystalline PbSe powders. As the first step, nanocrystalline lead selenide was synthesized by simple chemical method at 80°C using lead nitrate [Pb(NO3)2] and sodium selenosulphate [Na2SeSO3] in the aqueous alkaline media. Ethylene Diamine Tetra acetic acid (0.1M) was used as a complexing agent to form stable complexes with metal ions. Later on, the lead selenide thin films were deposited on the degreased glass substrates under a vacuum of 10−5Torr at various substrate temperatures by thermal evaporation technique using the pre-synthesized nanocrystalline PbSe powders. X-ray diffraction results show the synthesized powders and the deposited PbSe films belong to cubic structure. A gradual reduction in optical bandgap of films was observed with increasing substrate temperatures, which revealed the crystallization of the films. These observations are corroborated by photoluminescence spectroscopy study. Changes in surface morphology of the films with respect to substrate temperature were studied by high resolution scanning electron microscopy and atomic force microscopy. Electrical study infers the deposited films are of p-type semiconducting nature.

Cu nanoparticles induced structural, optical and electrical modification in PVA

Cu nanoparticles were synthesized in PVA matrix by chemical reduction of cupric nitrate with hydrazine hydrate. Structural characterization of synthesized Cu–PVA nanocomposite was carried out using UV–Visible Spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Appearance of characteristic surface plasmon resonance peak of Cu nanoparticles at 591 nm in absorption spectra of Cu–PVA colloidal solution confirms the formation of Cu nanoparticles. TEM investigation indicates that Cu nanoparticles of size 16 nm are formed in PVA matrix which are in agreement with the size obtained using X-ray diffraction. Morphology of Cu–PVA nanocomposite was further confirmed using SEM. Analysis of UV–Visible absorption and reflection data indicates towards the reduction in optical band gap and increase in refractive index of the resulting nanocomposite. The synthesized Cu–PVA nanocomposite has been found to be more conducting than PVA as ascertained using I–V studies. The decrease in band gap and increase in conductivity can be correlated due to the formation of localized electronic states in PVA matrix due to insertion of Cu nanoparticles. Photoluminescence measurements showed strong blue visible emission peak at 450 nm at different excitation wavelengths in the range of 220–245 nm.

Solution Growth and Optical Characterization of Thin Films with ZnO1-xSx and ZnO Nanorods in Core-Shell like Nanostructure for Solar Cell Application

ABSTRACTZnO films with a nanostructure dominated by 150-200 nm size highly c-axis oriented nanorod arrays were deposited by hydrothermal synthesis over surface activated quartz substrates. Sulfur infiltration and growth of ZnO1-xSx over ZnO nanorods was carried out by chemiplating process using slow hydrolysis of thiourea solution at 95°C. Formation of ZnO1-xSx nanocrystals of 20-30 nm size over (0001) facets of the ZnO rods is shown. With progressive growth of ZnO1-xSx nanocrystal and full ZnO nanorod coverage, the formation ZnO/ZnO1-xSx core –shell nanostructure is realized. X-ray photoelectron spectroscopy analysis shows chemical shifts in O1s and S2p spectra confirming the formation of ZnO1-xSx (0.1≤x≤0.2) nanocrystal shell. Reduction in optical band gap from a 3.24 eV for ZnO nanorod core to 2.78 eV for the ZnO1-xSx shell is consistent with the band gap bowing effect due to sulfur addition over the ZnO nanorod surface.