First-principles study of electronic structures and optical properties of Cu, Ag, and Au-doped anatase TiO2

We perform first-principles calculations to investigate the band structure, density of states, optical absorption, and the imaginary part of dielectric function of Cu, Ag, and Au-doped anatase TiO₂ in 72 atoms systems. The electronic structure results show that the Cu incorporation can lead to the enhancement of d states near the uppermost of valence band, while the Ag and Au doping cause some new electronic states in band gap of TiO₂. Meanwhile, it is found that the visible optical absorptions of Cu, Ag, and Au-doped TiO₂, are observed by analyzing the results of optical properties, which locate in the region of 400-1000 nm. The absorption band edges of Cu, Ag, and Au-doped TiO₂ shift to the long wavelength region compared with the pure TiO₂. Furthermore, according to the calculated results, we propose the optical transition mechanisms of Cu, Ag, and Au-doped TiO₂. Our results show that the visible light response of TiO₂ can be modulated by substitutional doping of Cu, Ag, and Au.     

Titanium dioxide nanoparticles prepared by laser pyrolysis: Synthesis and photocatalytic properties

TiO₂ nanoparticles were synthesized via the laser pyrolysis of titanium tetrachloride-based gas-phase mixtures. In the obtained nanopowders, a mixture of anatase and rutile phases with mean particle size of about 14 nm was identified. Using the thermal heated laser nanopowders, mechanically stable films were produced by immobilizing titania nanopowders on glass substrates (the doctor blading method followed by compression). The photocatalytic activity of the prepared films was tested by the degradation of 4-chlorophenol in an aqueous solution under UV-illumination. By referring to known commercial samples (Degussa P25) similarly prepared, higher photocatalytic efficiency was found for the laser-prepared samples.     

Magnetoelectric coupling in RMn2O5 multiferroic: a Monte Carlo simulation

Magnetoelectric coupling in RMn₂O₅ (with R?=?non magnetic) multiferroics have been studied using the Monte Carlo simulation. The variation of magnetization and the polarization of RMn₂O₅ multiferroic have been determined. The system undergoes a magnetic transition at T_N and a further reduction of the temperature leads to a ferroelectric transition at T_C?<_? T_N depending on the coupling strength. Magnetic and ferroelectric hysteresis loops are obtained for several temperatures values. Variation of polarization with the external magnetic field of RMn₂O₅ has been given. Variation of polarization and magnetization with the electric field of RMn₂O₅ has been obtained.     

Concentration-dependent electronic structure and optical absorption properties of B-doped anatase TiO2

The concentration-dependent electronic structures and optical properties of B-doped anatase TiO₂ have been calculated using the density functional theory. The calculated results indicate that the electronic structures of B-doped TiO₂ have changed compared with those of pure TiO₂, which is mainly due to the new midgap states induced by B doping. As to the optical properties, we calculate the imaginary part of dielectric function ε₂(ω) and optical absorption spectra of pure and B-doped TiO₂. Two transitions E₁ and E₂ emerged after B doping. The intensity of absorption is enhanced by B doping both in the UV and visible regions. According to the results of imaginary part of dielectric function ε₂(ω) and DOS, it can be concluded that the two optical transitions correspond to the transitions from the O 2p states in the top of valence band to the midgap states and from the midgap states to the Ti 3d states in the bottom of conduction band, respectively. These results have important implications for the further development of photocatalytic materials.     

Influence of dopant level on structural, optical and magnetic properties of Co-doped anatase TiO2 nanoparticles

Cobalt-doped TiO₂ nanoparticles were synthesized by sol-gel method. The associated structural, optical, compositional and magnetic properties of the nanoparticles as a function of cobalt concentration have been systematically studied. The X-ray powder diffraction reveals that all samples have pure anatase phase tetragonal system and the lattice parameter analysis indicated that Co ions may substitute into the lattice of TiO₂. The average particle size is 15 nm, when found through transmission electron microscope. Optical spectroscopy measurement showed that the bandgap value decreases upon increasing Co concentration. The magnetic measurements revealed that the enhanced room temperature ferromagnetism (RTFM) strongly depends on the doping content.     

Preparation and characterization of nitrogen-doped TiO2 nanoparticles by the laser pyrolysis of N2O-containing gas mixtures

Nitrogen-doped TiO₂ nanoparticles have been prepared by the IR laser pyrolysis technique. A sensitized mixture of TiCl₄ (vapors) and N₂O was used as titanium and nitrogen precursors, respectively. The structural properties of the resultant N-doped nanoparticles such as the phase formation and the average particle size and distributions were investigated by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. The phase composition varied from almost pure anatase to mixtures of rutile and anatase. A decrease of the mean particle diameters from about 18 nm in case of the almost pure anatase sample to about 13 nm in case of the anatase-rutile mixture is observed. XPS analysis suggests and interstitial character of the doping process.     

Structural, optical and electrochemical properties of TiO2 thin films grown by APCVD method

Atmospheric pressure chemical vapor deposition (APCVD) of TiO₂ thin films has been achieved onto glass and onto ITO-coated glass substrates, from the reaction of TiCl₄ with ethyl acetate (EtOAc). The effect of the synthesis temperature on the optical, structural and electrochemical properties was studied through spectral transmittance, X-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS) measurements. It was established that the TiO₂ films deposited onto glass substrate, at temperatures greater than 400 °C grown with rutile type tetragonal structure, whereas the TiO₂ films deposited onto ITO-coated glass substrate grown with anatase type structure. EIS was applied as suitable method to determine the charge transfer resistance in the electrolyte/TiO₂ interface, typically found in dye-sensitized solar cells.     

Microstructural, optical and photocatalytic properties of CdS doped TiO2 thin films

CdS doped TiO₂ thin films (with CdS content=0, 3, 6, 9 and 12 at%) were grown on glass substrates. The X-ray diffraction analysis revealed that the films are polycrystalline of monoclinic TiO₂ structure. The microstructure parameters of the films such as crystallite size (D_ν) and microstrain (e) are calculated. Both the crystallites size and the microstrain are decreased with increasing CdS content. The optical constants have been determined in terms of Murmann's exact equations. The refractive index and extinction coefficient are increased with increasing CdS content. The optical band gap is calculated in the strong absorption region. The possible optical transition in these films is found to be an allowed direct transition. The values of E_g^opt are found to decrease as the CdS content increased. The films with 3 at% CdS content have better decomposition efficiency than undoped TiO₂. The films with 6 at% and 9 at% CdS content have decomposition efficiency comparable to that of undoped TiO₂, although they have lower band gap. The CdS doped TiO₂ could have a better impact on the decomposing of organic wastes.     

Preparation and characterization of SnO2 nanoparticles using high power pulsed laser

Tin dioxide (SnO₂) nanoparticles having 3 nm size were synthesized by irradiating pure tin metal using high power Nd:YAG laser in deionized water. Formation of nano-SnO₂ crystallites was confirmed by X-ray diffraction (XRD) and AFM study. UV-vis absorption spectral studies showed a peak at 240 nm. FTIR spectrum showed a band in the range of 400-700 cm⁻¹ which was assigned to Sn-O antisymmetric vibrations. Photoluminescence spectrum of synthesized SnO₂ nanoparticles showed peak corresponding to 3.175, 2.901 and 2.613 eV respectively.     

Optical properties of ITO/TiO2 single and double layer thin films deposited by RPLAD

Indium tin oxide (ITO) and titanium dioxide (TiO₂) single layer and double layer ITO/TiO₂ films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser for applications in dye-sensitized solar cells (DSSCs). The films were deposited on SiO₂ substrates either at room temperatures (RT) or heated to 200-400 °C. Under optimized conditions, transmission of ITO films in the visible (vis) range was above 89% for films produced at RT and 93% for the ones deposited at higher temperatures. Increasing the substrate temperature from RT to 400 °C enhances the transmission of TiO₂ films in the vis-NIR from about 70% to 92%. High transmission (≈90%) was observed for the double layer ITO/TiO₂ with a transmission cut-off above 900 nm. From the transmission data, the energies gaps (E_g), as well as the refractive indexes (n) for the films were estimated. n ≈ 2.03 and 2.04, respectively for ITO films and TiO₂ film deposited at 400 °C in the visible region. Post-annealing of the TiO₂ films for 3 h at 300 and 500 °C was performed to enhance n. The refractive index of the TiO₂ films increases with the post-annealing temperature. The direct band gap is 3.6, 3.74 and 3.82 eV for ITO films deposited at RT, 200, and 400 °C, respectively. The TiO₂ films present a direct band gap of 3.51 and 3.37 eV for as deposited TiO₂ films and when annealed at 400 °C, respectively. There is a shift of about 0.1 eV between ITO and ITO/TiO₂ films deposited at 200 °C. The shift decreases by half when the TiO₂ film was deposited at 400 °C. Post-annealing was also performed on double layer ITO/TiO₂.     

ZnO1-xCx稀磁半导体的磁特性的第一性原理和蒙特卡罗研究

利用第一性原理计算得到C掺杂ZnO的电子结构,发现系统具有半金属的电子结构.从态密度的分析可以看到Zn-3d和C-2p电子具有强烈的杂化作用,这是体系具有相对稳定铁磁基态的原因.利用第一性原理得到的磁性耦合强度并结合蒙特卡罗模拟得到了C掺杂浓度为555%,833%,125%的ZnO_1-xC_x分别具有210 K,260 K,690 K的居里温度.同时,详细地分析了C掺杂引起的电子转移和C,Zn,O的s,p和d电子的自旋向上和自旋向下电子数的变化.通过比较研究,发现ZnO_1-xC_x的局域磁矩主要来源于Zn-3d 电子和C-2p 电子之间的相互作用,而局域磁矩耦合倾向于RKKY耦合. 关键词： 1-xC_x')" href="#">ZnO_1-xC_x 磁性 第一性原理 蒙特卡罗模拟     

A comparison between optical properties of TiO2 nanowires obtained by EMA method and experiment

Optical properties of TiO₂ nanowires, synthesized by two-step thermal evaporation process, have been studied experimentally and theoretically. Based on the theoretical method optical constants of nanowires have been calculated with the use of the effective medium approximation (EMA). As evidenced by X-ray diffraction patterns our synthesized nanowires, whose diameters and lengths were within the ranges of 50-90 nm and 500-1500 nm, respectively, were found to be crystalline rutile TiO₂ with the major refraction being along the (1 1 0) direction. The experimental data of the reflectance of TiO₂ nanowires has been obtained using spectrometer in wavelength 250-800 nm, and then, compared with the spectrum of reflectance predicted by the EMA theoretical model. Our measured experimental optical data has been found to be in good accord with our predicted results spectrum with the use of the EMA modeling; this agreement indicates that our estimation of the volume fraction from atomic force microscopy (AFM) data was accurate.     

One-dimensional self-assembly of C60 molecules on periodically wrinkled graphene sheet: A Monte Carlo approach

Periodically wrinkled graphene sheet is of interest as a building block to develop nanoelectronic devices. This work presents that periodically wrinkled graphene sheet can be applied to a pattern, to form one-dimensionally well-ordered C₆₀ molecules, via Monte Carlo simulations using the data obtained from atomistic calculations. Since the valleys of a sinusoidal graphene surface provide energetic ground sites for absorbed C₆₀ molecules, their motions seeking stable positions lead to one-dimensional self-assembly. The size of the wrinkles, the density of adsorbed C₆₀ molecules, and the temperature are very important parameters to obtain a one-dimensional C₆₀ molecules array. We estimate high one-dimensional diffusion coefficients of C₆₀ molecules on the wrinkled graphene surface. Our results can provide a possible approach to make a quantum information array, based on endohedral fullerenes and a graphene quantum dot array, by transforming C₆₀ molecules to graphene nanoflakes.     

Synthesis of anatase TiO2 nanowires by modifying TiO2 nanoparticles using the microwave heating method

Anatase TiO₂ nanowires with a diameter of 5-10 nm and length of 500 nm to 2 μm have been successfully synthesized by modifying TiO₂ nanoparticles (P25) using the microwave heating method. The microwave power, reaction pressure, and reaction time for the synthesis of TiO₂ nanowires were 500 W, 0.5-3.0 MPa (corresponding to a temperature range of 175-260), and 40-70 min, respectively. X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and the BET techniques were used to investigate the phase structures, morphologies, and specific surface areas of the TiO₂ nanowires. The effects of reaction time, pressure, and different post-treatment processes on the microstructures of TiO₂ nanowires were discussed. It has been shown that the microwave heating method is efficient in transforming TiO₂ nanoparticles to anatase TiO₂ nanowires.     

Electronic structures and optical properties of rutile TiO2 with different point defects from DFT + U calculations

The electronic states and formation energies of four types of lattice point defects in rutile TiO₂ are studied using the first-principles calculations. The existence of oxygen vacancy leads to a deep donor defect level in the forbidden band, while the Ti interstitial forms two local states. It is predicted that oxygen vacancy prefers to combine with Ti-interstitial to form V_O–Ti_i dimer by a partial 3d electron transfer from the Ti_i to its neighboring V_O. The charge distribution between a Ti interstitial and its neighboring Ti ions partially shields the Coulomb interactions. Lastly, optical properties of these defective lattices are discussed.     

Structural and optical properties of ZnO thin films on (111) CaF2 substrates grown by magnetron sputtering

ZnO thin films were grown on (111) CaF₂ substrates by magnetron sputtering at room temperature. Structural and optical properties of the ZnO thin films were studied. XRD analysis showed that the ZnO thin films had the (002) preferential orientation. The transmittance of ZnO thin films was over 80% in the visible range. The optical band gap of the ZnO thin films was 3.26 eV. The optical constants (n,k)$(n,k)$ of the ZnO thin films in the wavelength range 300–1000 nm were obtained by infrared spectroscopic ellipsometry measurement. PL spectra of ZnO thin films showed strong UV near-band-edge emission peak at 376.5 nm and weak visible red emission at 643.49 nm using He–Cd laser as the light source, using a synchrotron radiation light source PL spectra showed three emission peak at 320 nm, 410 nm and 542 nm respectively.     

Effect of nickel doping on structural, optical and electrical properties of TiO2 nanoparticles by sol-gel method

Nickel-doped anatase TiO₂ nanoparticles have been prepared by sol-gel method. The X-ray powder diffraction study reveals that all the prepared samples have pure anatase phase tetragonal system. The average crystallite size of the prepared sample is 14 nm, when found through transmission electron microscope. A strong frequency dependence of both dielectric constant (?′) and dielectric loss (tan δ) were observed for various dopant levels at room temperature in the frequency range of 42 Hz to 5 MHz. At low frequency, the piling up of mobile charge carriers at the grain boundary produces interfacial polarization giving rise to high dielectric constant. The asymmetric shape of frequency dependence of the dielectric loss for the primary relaxation process is observed for each concentration. From the ac conductivity studies, the reduction in conductivity may arise due to the decreasing particle with the increase in Ni-dopant level.     

Influence of Fe ions in characteristics and optical properties of mesoporous titanium oxide thin films

Fe-doped mesoporous titanium dioxide (M-TiO₂-Fe) thin films have been prepared on indium tin oxide (ITO) glass substrates by sol–gel and spin coating methods. All films exhibited mesoporous structure with the pore size around 5–9 nm characterized by small angle X-ray diffraction (SAXRD) and further confirmed by high resolution transmission electron microscopy (HRTEM). Raman spectra illustrated that lower Fe-doping contributed to the formation of nanocrystalline of M-TiO₂-Fe thin films. X-ray photoelectron spectroscopy (XPS) data indicated that the doped Fe ions exist in forms of Fe³⁺, which can play a role as e⁻ or h⁺ traps and reduce e⁻/h⁺ pair recombination rate. Optical properties including refractive indices/n, energy gaps/E_g and Urbach energy width/E₀ of the thin films were estimated and investigated by UV/vis transmittance spectra. The presence of Fe content extended the light absorption band and decreased the values of n, implying enhanced light response and performance on dye-sensitized solar cells (DSSC). The optimum Fe content in M-TiO₂-Fe thin films is determined as 10 mol%, for its compatibility of well crystalline and well potential electron transfer performance.     

Effect of temperature,electric and magnetic field on spin relaxation in single layer graphene: A Monte Carlo simulation study

In this article, we employ the semiclassical Monte Carlo approach to study the spin polarized electron transport in single layer graphene channel. The Monte Carlo method can treat non-equilibrium carrier transport and effects of external electric and magnetic fields on carrier transport can be incorporated in the formalism. Graphene is the ideal material for spintronics application due to very low Spin Orbit Interaction. Spin relaxation in graphene is caused by D'yakonov-Perel (DP) relaxation and Elliott-Yafet (EY) relaxation. We study effect of electron electron scattering, temperature, magnetic field and driving electric field on spin relaxation length in single layer graphene. We have considered injection polarization along z-direction which is perpendicular to the plane of graphene and the magnitude of ensemble averaged spin variation is studied along the x-direction which is the transport direction. This theoretical investigation is particularly important in order to identify the factors responsible for experimentally observed spin relaxation length in graphene.     

Influence of growth temperature of TiO2 buffer on structure and PL properties of ZnO films

A series of ZnO films with TiO₂ buffer on Si (1 0 0) substrates were prepared by DC reactive sputtering. Growth temperature of TiO₂ buffer changed from 100 °C to 400 °C, and the influence on the crystal structures and optical properties of ZnO films have been investigated. The XRD results show that the ZnO films with TiO₂ buffer have a hexagonal wurtzite structure with random orientation, and with the increase of growth temperature of TiO₂ buffer, the residual stresses were released gradually. Specially, the UV emission enhanced distinctly and FWHMs (full width half maximum) decreased linearly with the increasing TiO₂ growth temperature. The results all come from the improvement of crystal quality of ZnO films.