The Band Gap of BaPrO3 Studied by Optical and Electrical Methods

We report on measurements of the electrical and optical properties of BaPrO₃. The temperature dependences of the electrical conductivity σ and the Seebeck coefficient α of polycrystalline samples were studied over a wide temperature range (300°C–1050°C). At lower temperatures, the observed charge transport can be described as thermally activated hopping of electron‐based small polarons with an activation energy of 0.37 eV. An observed change in temperature dependence of both σ and α around 700°C was observed and interpreted as a transition from extrinsic to intrinsic carrier transport. The intrinsic conduction can be modeled with an apparent electrical band gap of ~2 eV. Optical absorption and emission spectroscopy in the UV–VIS–NIR range revealed a series of characteristic absorption thresholds and the type of optical transitions was identified by combining transmittance and diffuse‐reflectance spectroscopy methods. An absorption edge of indirect type with onset at 0.6 eV is attributed to small polaron effects. The higher lying absorption thresholds of direct origin positioned at around 1.8 and 3.8 eV are correlated with thermal activation parameters from electrical measurements and discussed in terms of the band gap of BaPrO₃.     

Modifications in the structural,morphological, optical properties of Ag45Se40Te15 thin films by proton ion irradiation for optoelectronics and nonlinear applications

This current work reports the 30 keV proton ion irradiation induced structural, morphological, and optical properties change in Ag₄₅Se₄₀Te₁₅ films at different fluences. The thin films were irradiated with different ion fluences, such as 5 × 10¹⁵ ions/cm²,1 × 10¹⁶ ions/cm² and 5 × 10¹⁶ ions/cm². The electronic loss (S_e) dominates over the nuclear loss (S_n) in proton irradiation. The X-ray diffraction study shows the phase transformation from amorphous to crystalline upon ion irradiation. The Raman analysis confirms the change in chemical and vibrational bonds due to structural alterations in the films. The surface morphology has been studied by field emission scanning electron microscopy and the composition of the films has been checked by the energy dispersive X-ray analysis. The particle size increased upon the increase in ion irradiation fluence. The surface roughness of the films has been studied by atomic force microscopy. The transmission data is used to calculate the linear optical parameters. The absorption edge shifts towards the high wavelength region inferring the reduction in the optical bandgap. The linear refractive index of the films increased with ion fluence. The optical density increased at the high wavelength region while the skin depth decreased with fluence. The carrier concentration per effective mass decreased while the plasma frequency increased with proton irradiation. The nonlinearity (χ ⁽³⁾ and n₂) values increased significantly with the increase in fluences. Such kind of materials with optimization in their optical parameters are primarily essential for cutting-edge photonic, optoelectronic, and nonlinear optical applications.     

Selective band gap manipulation of graphene oxide by its reduction with mild reagents

Graphene oxide (GO) can be used as an electron acceptor for polymeric solar cells but still band gap matching for electron donor and acceptor demands more study. The generation of the exciton in such materials is intimately related to the optical band gap. However, exciton dissociation is related to transport band gap that controls the device performance, particularly the open circuit voltage and short circuit current. Therefore, the modulation of the optical gap is useful because it results into tuning of the transport gap. The interest of the present work is to study the reduction of graphene oxide (GO) at room temperature, using environmental friendly reagents like glucose, fructose and ascorbic acid for the modulation of a band gap. It has been found that glucose and fructose function effectively only in presence of NH₄OH. Although ascorbic acid can reduce GO alone, NH₄OH speeds up the reaction. The optical band gap of GO can be reduced and tuned effectively from 2.7 eV to 1.15 eV.     

Effects of Si5+ Ion Irradiation on Poly(3-methyl thiophene) Films

Poly(3-methyl thiophene) has been synthesized by the chemical oxidation polymerization method using FeCl₃. The powder has been dissolved in CHCl₃ and thin films of thickness 2 µm are prepared on glass and Si substrates. The polymerization has been confirmed by FTIR spectrum. The films are irradiated by 60 MeV Si⁵⁺ ions at different fluences and modifications in optical, electrical and structural properties are studied. FTIR spectra show methyl group evolution after irradiation. The optical band gap decreases after irradiation and dc conductivity increases four orders of magnitude after irradiation at the highest fluence. The conduction mechanism has been found mainly by band conduction.     

Structural and electrical properties of lead free ceramic: Ba(La1/2Nb1/2)O3

Abstract

Abstract

Lead free perovskite Ba(La_1/2Nb_1/2)O₃ was prepared by conventional ceramic fabrication technique at 1375°C for 7?h in air atmosphere. The crystal symmetry, space group and unit cell dimensions were estimated using Rietveld analysis. X-ray diffraction analysis indicated the formation of a single phase monoclinic structure with space group P2/m. Energy dispersive X-ray analysis and scanning electron microscopy studies were carried to study the quality and purity of the compound. Permittivity data showed low temperature coefficient of capacitance (T _CC = 11%) up to 100°C. The circuit model fittings were carried out using the impedance data to find the correlation between the response of real system and idealised model electrical circuit. Complex impedance analyses suggested the dielectric relaxation to be of non-Debye type. The correlated barrier hopping model was employed to successfully explain the mechanism of charge transport in Ba(La_1/2Nb_1/2)O₃. The AC conductivity data were used to evaluate the density of states at Fermi level, minimum hopping length and apparent activation energy.     

Design of semiconductor ternary quantum dots with optimal optoelectronic function

The function of nanometer‐size quantum dots (QDs) of ternary compound semiconductors, such as In_xGa_1?xAs and ZnSe_1?xTe_x, used in the fabrication of optoelectronic and photovoltaic devices can be optimized by precise tuning of their electronic band gap through control of the QD composition (x) and diameter. Results on compositional distributions in ternary QDs and how they affect the QDs' electronic band gap are reported. A hierarchical modeling approach is followed that combines first‐principles density functional theory calculations and classical Monte Carlo simulations with a continuum model of species transport in spherical nanocrystals. In certain cases, the model predicts the formation of core/shell‐like structures with shell regions rich in the surface segregating species. A systematic parametric analysis generates a database of transport properties, which can be used to design post‐growth thermal annealing processes that enable the development of thermodynamically stable QDs with optimal electronic properties grown through simple one‐step colloidal synthesis techniques. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3223–3236, 2013     

Evaluation of structural properties of AZO conductive films modification using low-temperature ultraviolet laser annealing

In this study, an ultraviolet (355 nm) laser processing system was developed to anneal aluminum-doped zinc oxide (AZO) thin films at room temperature in an air atmosphere; in this system, two key parameters, laser fluence and annealing speed, were varied. The structural properties of the films were thoroughly examined using field emission scanning electron microscopy, atomic force microscopy, and X-ray diffraction (XRD). The results showed that the laser fluence not only influenced the structural properties of the films, but also improved the crystallinity of the films after the laser annealing process, with minimal changes in the thickness of the films and the concentration of the elements in the films. The root mean square surface roughness (R_rms) of the films gradually increased as the laser fluence increased. Moreover, according to the XRD pattern of the films, the intensity of the main peak corresponding to the (002) direction increased as the laser fluence increased. The average crystallite size (20 nm) of the annealed films, determined using the Scherrer equation, was smaller than that of the as-deposited thin film (22 nm) due to the low temperature effect in the laser annealing process.     

Electrical and optical study of Cu(In, Ga)Se2 co-evaporated thin films

Co-evaporation technique from three sources was used to prepare Cu(In, Ga)Se₂ polycrystalline thin films for photovoltaic conversion. Their conductivity was studied in the range 20–300 K. The grain boundary scattering mechanism is mainly responsible for the diffusion process in the latter materials. In the low temperature region, we interpret the data in terms of Mott law and the analysis is very consistent with the variable range hopping. However, thermoionic emission is predominant at high temperatures. When the conductivity deviates from the classical grain boundary conduction models, inhomogeneity is then considered and parameters such as the standard deviation and the mean potential barrier height are derived. Transmittance measurements yielded band gap values of 1.07 and 1.64 eV for CuInSe₂ and CuGaSe₂, respectively.     

Evolution in the Electronic Structure of Polymer‐derived Amorphous Silicon Carbide

The electronic structure of polymer‐derived amorphous silicon carbide pyrolyzed at different temperatures was investigated by combining measurements of their temperature‐dependent conductivity and optical absorption. By comparing the experimental results with theoretical models, the parameters such as conduction band, band‐tail, defect energy, and Fermi energy were determined. The results revealed that band gap and band‐tail width decreased with increasing pyrolysis temperature. Furthermore, it was found that electrons transport followed a band‐tail hopping mechanism, rather than variable range hopping. These results were discussed in accordance with the microstructural evolutions of the material.     

High energy (150 MeV) Fe11+ ion beam induced modifications of physico-chemical and photoluminescence properties of high-k dielectric nanocrystalline zirconium oxide thin films

This work presents the influence of dominated electronic energy loss over nuclear energy loss induced by swift heavy ion (SHI) irradiation on the physico-chemical, optical and other properties of RF grown zirconium oxide (ZrO₂) thin films. For this purpose, thin films of ZrO₂ grown on glass substrate were irradiated by 150 MeV Fe¹¹⁺ ions with a range of fluence from 2E12 to 5E13 ions/cm² to understand the mechanism of induced modifications and defects generation. The XRD results confirmed that the virgin and irradiated ZrO₂ thin films were crystalline in nature with monoclinic and tetragonal structure. The crystallite size varied from 19.93 nm to 46.43 nm with varying ion fluence. Strain, dislocation density and stacking fault were used to investigate the changes in structural parameters. Tauc's plot method was employed for the quantitative evaluation of optical energy band gap (E_g) that exist in the range of 4.45–4.62 eV. The transmittance (%) of the virgin and Fe¹¹⁺ ions irradiated samples was determined in the range of 35.69–66.09% using UV–Vis. spectroscopy. Further, the refractive index was determined using different methods significantly depends on the optical band gap. The broad PL emission peaks were obtained at 375 nm and 440 nm with the excitation wavelength (λ_ex.) of 300 nm. The variation in PL intensity with increasing ion fluence was attributed to the creation or annihilation of primary or complex defects. FTIR spectroscopy was employed for the analysis of chemical modifications in vibrational bonds of samples and the band obtained 660 cm⁻¹was assigned to the asymmetrically coupled Zr–O–Zr stretching which presents the strong vibration in samples. The band intensity increased up to the fluence 5E12 ions/cm² and decreased at a higher fluence of 1E13 ions/cm². Rutherford backscattering spectroscopy technique was used to determine the thickness (165 nm) of the samples.     

Structural investigation of low energy ion irradiated Al2O3

The present study is undertaken to investigate the ion irradiation impact on the structural properties of α-Al₂O₃ which is one of the potential insulator used for ITER-like reactors. 300 keV Ar⁶⁺ ion beam with varying fluence is used to irradiate Al₂O₃ polycrystalline material in bulk form. Structural damage to α-Al₂O₃ due to ion irradiation is investigated by grazing incidence XRD (GIXRD), micro-RAMAN and photoluminescence spectroscopy. Further, the surface morphology of irradiated and pristine samples is studied using atomic force microscopy. GIXRD data reveal the radiation induced damage at low fluence and structural improvement at higher fluence up to 1 × 10¹⁶ ions/cm². Annealing effect is explained by temperature rise during irradiation and due to partial grain growth of Al₂O₃ at 1 × 10¹⁶ ions/cm². It is important to note that there is no sign of amorphization even at the highest fluence 1 × 10¹⁶ ions/cm² used in this study. The explanation is supported by the Raman characterization results. Detailed peak assessment of photoluminescence spectra is presented. Photoluminescence characterization results are corroborated with XRD results. The present study reveals the radiation hardness of polycrystalline Al₂O₃ material in the low energy ion regime.     

Manual turbostratic stacked graphene transistor: A study on electrical properties and device potential

This study investigated the potential of turbostratic stacking of graphene few-layers produced using the consecutive electrochemical delamination method for electronic applications. The temperature dependence of electrical conductivity was measured to identify the transport mechanisms and band overlap. By lowering the temperature from 298 to 20 K, it was shown that these highly disordered structures follow nearest neighbor hopping through the variable range hopping mechanism. Variations in band overlap for samples versus carrier concentration were extracted and show that trilayer graphene has the best electrical properties at a mobility of about 1000 cm²/V s and the lowest band overlap at 28.5 meV which is promising structure for optoelectronic applications.     

Fabrication of highly efficient TiO2/Ag/TiO2 multilayer transparent conducting electrode with N ion implantation for optoelectronic applications

Fabrication of highly conductive and transparent TiO₂/Ag/TiO₂ (referred hereafter as TAT) multilayer films with nitrogen implantation is reported. In the present work, TAT films were fabricated with a total thickness of 100 nm by sputtering on glass substrates at room temperature. The as-deposited films were implanted with 40 keV N ions for different fluences (1×10¹⁴, 5×10¹⁴, 1×10¹⁵, 5×10¹⁵ and 1×10¹⁶ ions/cm²). The objective of this study was to investigate the effect of N⁺ implantation on the optical and electrical properties of TAT multilayer films. X-ray diffraction of TAT films shows an amorphous TiO₂ film with a crystalline peak assigned to Ag (111) diffraction plane. The surface morphology studied by atomic force microscopy (AFM) and field emission scanning electron microscope (FESEM) revealed smooth and uniform top layer of the sandwich structure. The surface roughness of pristine film was 1.7 nm which increases to 2.34 nm on implantation for 1×10¹⁴ ions/cm² fluence. Beyond this fluence, the roughness decreases. The oxide/metal/oxide structure exhibits an average transmittance ~80% for pristine and ~70% for the implanted film at fluence of 1×10¹⁶ ions/cm² in the visible region. The electrical resistivity of the pristine sample was obtained as 2.04×10⁻⁴ Ω cm which is minimized to 9.62×10⁻⁵ Ω cm at highest fluence. Sheet resistance of TAT films decreased from 20.4 to 9.62 Ω/□ with an increase in fluence. Electrical and optical parameters such as carrier concentration, carrier mobility, absorption coefficient, band gap, refractive index and extinction coefficient have been calculated for the pristine and implanted films to assess the performance of films. The TAT multilayer film with fluence of 1×10¹⁶ ions/cm² showed maximum Haacke figure of merit (FOM) of 5.7×10⁻³ Ω⁻¹. X-ray photoelectron spectroscopy (XPS) analysis of N 1s and Ti 2p spectra revealed that substitutional implantation of nitrogen into the TiO₂ lattice added new electronic states just above the valence band which is responsible for the narrowing of band gap resulting in the enhancement in electrical conductivity. This study reports that fabrication of multilayer transparent conducting electrode with nitrogen implantation that exhibits superior electrical and optical properties and hence can be an alternative to indium tin oxide (ITO) for futuristic TCE applications in optoelectronic devices.     

Polymer electronic materials: a review of charge transport

This article presents an overview of the charge transport phenomenon in semiconducting polymer materials. In these disordered systems both intrinsic and extrinsic parameters play significant roles. In general, π‐electron delocalization, interchain interaction, band gap, carrier density, extent of disorder, morphology and processing of materials determine the electrical and optical properties. The chemical structure, especially the role of side groups, is quite important in both physical and processing properties. The nature of charge carriers and their role in charge transport depend on the structure and morphology of the system. Hence in several semiconducting polymer devices, the correlations among structure, morphology and transport are rather strong. The dependence of carrier mobility on temperature and electric field needs to be understood in the framework of competing models based on carrier hopping, trapping/detrapping and tunneling. Exactly what determines the dispersive/nondispersive, polaronic and correlative transport regimes is yet to be quantified. An understanding of the carrier mobility in semiconducting polymers is necessary to optimize the performance of polymeric electronic devices. Copyright © 2006 Society of Chemical Industry     

n-type conductivity of phosphorus-doped homoepitaxial single crystal diamond on (001) substrate

The macroscopic surface morphology and crystallinity of (001) n-type diamond films, which have been quite recently achieved by P-doping using plasma-enhanced chemical vapor deposition technique, were studied. The observation of diffraction spots, streaks, and Kikuchi patterns in reflection high energy electron diffraction analysis indicated that the surface smoothness and the crystallinity were fine. Regarding the electrical properties of (001) n-type diamond films, Hall-effect measurements over a wide temperature range from 260 to 1000 K were investigated. The conduction band transport without the effect of hopping transport was confirmed within this experimental temperature range.     

Atomic order-disorder engineering in the La2Zr2O7 pyrochlore under low energy ion irradiation

Cation and anion disordering affect the structural and electronic properties of the isometric A₂B₂O₇ pyrochlore materials. Here, we report a study on the structural response of La₂Zr₂O₇ at two different temperatures (300 K and ~88 K) as a function of ion fluence (1 × 10¹³, 5 × 10¹³, and 1 × 10¹⁴ ions/cm²). The effect of ion fluence and irradiation temperature on the structural properties have been investigated using the grazing angle x-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy. GIXRD results confirmed that the weakening/broadening of the diffraction peaks and lattice volume expansion increases monotonically as a function of ion fluence at both the temperatures and are more pronounced at ~88 K. The cation and anion disordering appear to be ion fluence and irradiation temperature-dependent. Raman spectroscopy shows that the atomic disordering is more pronounced with enhanced ion fluence and revealed the involvement of the X_48f parameter in the enhancement of disordering in the system. The HRTEM analysis revealed that the deterioration in the atomic ordering (amorphization) is significantly more pronounced at ~88 K. The qualitative analysis of cation/anion disordering and structural deformation revealed that irradiation parameters play a crucial role in developing and altering the properties of the pyrochlore materials for the technological applications.     

Synthesis of copper chloride and cobalt chloride doped polyanilines and their magnetic and alternating‐current transport properties

Polyaniline (PANI) was synthesized by the well‐known oxidative polymerization of aniline with ammonium peroxodisulfate as the oxidant. The morphological, structural, thermal, optical, magnetic, and electrical properties were characterized with scanning electron microscopy, X‐ray diffraction, Fourier transform infrared, thermogravimetric analysis, differential scanning calorimetry, ultraviolet–visible spectroscopy, room‐temperature magnetic measurements, and low‐temperature electrical transport measurements by the standard four‐probe method. Greater thermal stability and crystallinity were observed in doped PANI versus pure PANI. Magnetic measurements showed that the magnetic susceptibility was field‐dependent. Positive and negative susceptibility values were observed. This may have been due to the interactions of magnetic ions among interchains or intrachains of the polymer matrix. The alternating‐current (ac) conductivity was measured in the temperature range of 77–300 K in the frequency range of 20 Hz to 1 MHz. The frequency‐dependent real part of the complex ac conductivity was found to follow the universal dielectric response: σ′(f) ∝ f^s [where σ′(f) is the frequency‐dependent total conductivity, f is the frequency, and s is the frequency exponent] The trend in the variation of the frequency exponent with temperature corroborated the fact that correlated barrier hopping was the dominant charge‐transport mechanism for PANI–CoCl₂. An anomalous dependence on temperature of the frequency exponent was observed for PANI–CuCl₂. This anomalous behavior could not be explained in terms of existing theories. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Substrate Temperature Dependent Surface Morphology and Photoluminescence of Germanium Quantum Dots Grown by Radio Frequency Magnetron Sputtering

The visible luminescence from Ge nanoparticles and nanocrystallites has generated interest due to the feasibility of tuning band gap by controlling the sizes. Germanium (Ge) quantum dots (QDs) with average diameter ~16 to 8 nm are synthesized by radio frequency magnetron sputtering under different growth conditions. These QDs with narrow size distribution and high density, characterized using atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM) are obtained under the optimal growth conditions of 400 °C substrate temperature, 100 W radio frequency powers and 10 Sccm Argon flow. The possibility of surface passivation and configuration of these dots are confirmed by elemental energy dispersive X-ray (EDX) analysis. The room temperature strong visible photoluminescence (PL) from such QDs suggests their potential application in optoelectronics. The sample grown at 400 °C in particular, shows three PL peaks at around ~2.95 eV, 3.34 eV and 4.36 eV attributed to the interaction between Ge, GeO_x manifesting the possibility of the formation of core-shell structures. A red shift of ~0.11 eV in the PL peak is observed with decreasing substrate temperature. We assert that our easy and economic method is suitable for the large-scale production of Ge QDs useful in optoelectronic devices.     

Semiconductor-conductor transition of pristine polymer-derived ceramics SiC pyrolyzed at temperature range from 1200°C to 1800°C

This paper studies the effect of pyrolysis temperature on the semiconductor-conductor transition of pristine polymer-derived ceramic silicon carbide (PDC SiC). A comprehensive study of microstructural evolution and conduction mechanism of PDC SiC pyrolyzed at the temperature range of 1200°C-1800°C is presented. At relatively lower pyrolysis temperatures (1200°C-1600°C), the carbon phase goes through a microstructural evolution from amorphous carbon to nanocrystalline carbon. The PDC SiC samples behave as a semiconductor and the electron transport is governed by the band tail hopping (BTH) mechanism in low pyrolysis temperature (1300°C); by a mixed mechanism driven by band tail hopping and tunneling at intermediate temperature (1500°C). At higher pyrolysis temperatures (1700°C-1800°C), a percolative network of continuous turbostratic carbon is formed up along the grain boundary of the crystallized SiC. The samples demonstrate metal-like conductive response and their resistivity increases monotonically with the increasing measuring temperature.     

Ion induced modification of bandgap and CIE parameters in Y2O3:Dy phosphor

This paper reports the comparative investigation on structural and optical modifications of Y₂O₃:Dy³⁺ phosphor after 150 MeV Ni⁷⁺ and 120 MeV Ag⁹⁺ swift heavy ions irradiation in the fluence range 1×10¹¹–1×10¹³ ions/cm². XRD and TEM studies confirm the loss of crystallinity of ion irradiated phosphors. Diffuse reflectance spectrum shows a blue shift in the absorption band resulting in an increase in band gap after ion irradiation. An increase in the intensity of photoluminescence peaks without any shift in the peak positions was observed with ion fluence. The color coordinates of ion irradiated phosphors approach the white light region with the increase of ion fluence.