Optical Absorption Tail Research Articles

Nanoparticles of ZrO2:xSr2+ produced by the sonochemical method: Structural, morphological, and photoluminescent characterization for optoelectronic applications

The pursuit of efficient materials for technological applications is essential. Among these materials, zirconia emerges as a promising candidate with potential for several applications, from applications related to mechanical resistance to optoelectronic applications. By introducing dopants into this compound, it becomes feasible to modulate its inherent properties. In this study, ZrO2 doped with strontium was synthesized for the first time via the sonochemical method at different percentages in mol (1, 2, 4, and 8 mol%). Analysis via X-ray diffraction (XRD) and Raman spectroscopy unveiled the formation of both monoclinic and tetragonal phases, with the latter being stabilized through the incorporation of the dopant element. In addition to observing irregular morphology through scanning electron microscopy (SEM-FEG), a reduction in particle size was also evidenced following the introduction of Sr2+ ions, a phenomenon corroborated via Rietveld refinement. The reflectance spectra of the doped samples exhibited an optical absorption tail characterized by the Urbach energy (EU), indicating an increase in structural disorder in the samples. The photoluminescence (PL) results indicated that the introduction of Sr2+ ions into the ZrO2 matrix induced modifications in the characteristics of the spectrum profile. The phase transformation observed in XRD was reflected in the changes observed in the photoluminescent behavior of the samples, which emitted light between the blue and green regions.

Opto-Electronic Characterization of Photocatalysts Based on p,n-Junction Ternary and Quaternary Mixed Oxides Semiconductors (Cu2O-In2O3 and Cu2O-In2O3-TiO2)

Semiconductor materials are the basis of electronic devices employed in the communication and media industry. In the present work, we report the synthesis and characterization of mixed metal oxides (MOs) as p,n-junction photocatalysts, and demonstrate the correlation between the preparation technique and the properties of the materials. Solid-state UV-visible diffuse reflectance spectroscopy (UV-VIS DRS) allowed for the determination of the light absorption properties and the optical energy gap. X-ray photoelectron spectroscopy (XPS) allowed for the determination of the surface speciation and composition and for the determination of the valence band edge. The opto-electronic behavior was evaluated measuring the photocurrent generated after absorption of chopped visible light in a 3-electrode cell. Scanning electron microscopy (SEM) measurements allowed for auxiliary characterization of size and morphology, showing the formation of composites for the ternary Cu2O-In2O3 p,n-mixed oxide, and even more for the quaternary Cu2O-In2O3-TiO2 MO. Light absorption spectra and photocurrent-time curves mainly depend upon the composition of MOs, while the optical energy gap and defective absorption tail are closely related to the preparation methodology, time and thermal treatment. Qualitative electronic band structures of semiconductors are also presented.

Резонансное отражение света оптической решеткой экситонов, сформированной 100 квантовыми ямами InGaN

Optical properties of a structure with a periodic system of 100 InGaN quantum wells (QWs) separated by non-tunneling GaN barriers have been investigated at room temperature. The structure periodicity corresponded to the Bragg diffraction condition at the frequency of the QW excitons. Numerical modeling using transfer matrices gave a quantitatively accurate fit of the experimental results. The model included the resonance response of A, B, and C excitons in QWs and an optical absorption tail in the barriers and buffer layer. We have determined the radiative and non-radiative broadening of the excitons in the InGaN QWs.

Preparations of Organo-Lead Halide Perovskite Layers in Humid Air Atmosphere and their Characteristics

In recent years, solar cells based on organometal halide perovskite (hybrid perovskite) materials have emerged as promising low-cost solar cells. These types of solar cells exhibit remarkable high power conversion efficiencies while offering a significant advantage on the simplicity of their fabrications. The formation of organo-metal halide perovskite layer, however, is very sensitive to some process parameters such as concentration, temperature, and humidity. Here, we report the preparation of this perovskite crystal layer of CH3NH3PbI3 (MAPbI3) under relatively humid air atmosphere, which is a common condition in tropical countries, and understanding its effect on the resulted perovskite layer characteristics. We found that the humidity leads to the formation of a black color layer on the top of dark brown color perovskite layer. This black color layer caused the appearance of optical absorption tail extending to the near infrared region and showed poor x-ray diffraction (XRD) pattern. It also leads to poorer solar cell characteristics. The formation of this layer can be prevented by applying fast deposition-crystallization process with proper solvent and dropping time.

Dependences of the Optical Absorption, Ground State Energy Level, and Interfacial Electron Transfer Dynamics on the Size of CdSe Quantum Dots Adsorbed on the (001), (110), and (111) Surfaces of Single Crystal Rutile TiO2

Quantum dots (QDs) provide an attractive alternative sensitizer to organic dyes. However, there have been few reports on QD-sensitized solar cells (QDSCs) that have photovoltaic conversion efficiencies exceeding those of dye-sensitized solar cells. This is because of the lack of fundamental studies of QDs on conventional nanocrystalline metal oxide electrodes which possess much amount of heterogeneity. An important first step is an investigation of the dependences of the optical absorption, the ground state energy level, and the interfacial electron transfer (IET) on the size of QDs deposited on well characterized single crystal oxides. The present study focuses on a system of CdSe QDs adsorbed on the (001), (110), and (111) surfaces of single crystal rutile-TiO2. The optical absorption spectra, characterized using photoacoustic spectroscopy, were found to be independent of the surface orientation concerning the optical absorption edge. The exponential optical absorption tail (Urbach tail) suggests that t...

Hydrogen-treated hierarchical titanium oxide nanostructures for photoelectrochemical water splitting

Hierarchical titanium oxide nanostructures were synthesized by Pulsed Laser Deposition (PLD) and investigated as photoanodes for photoelectrochemical water splitting. An explorative combined approach to enhance TiO2 performance was based, on the one hand, on the employment of hydrogenation treatments with the aim of improving quantum efficiency and extending light absorption to the visible range; on the other hand, on the optimization of morphology and structure, to increase light harvesting and charge separation/transport. This approach was pursued by depositing at a fixed background pressure with variable oxygen content (to control the growth morphology and structure) and by annealing in a Ar/H2 mixture (in substitution of or in combination with air annealing), in order to induce crystallization to the anatase structure and reduction/hydrogenation of the material. Morphology, structure and optical properties were investigated by SEM, Raman spectroscopy, X-ray diffraction and UV–visible-IR spectroscopy. An optical absorption tail towards the visible range appeared after Ar/H2 annealing, without any significant modification of the nanoscale structure after the different thermal treatments. Photocurrent measurements under solar simulator illumination showed a noteworthy increase of photoresponse for Ar/O2-deposited samples with air annealing followed by Ar/H2 annealing. These findings can be ascribed to the combination between an improved charge transport of TiO2 deposited in low-O2 atmosphere and a hydrogenation effect on the nanostructures surface layers, leading to improved quantum efficiency.

Optical absorption of CdSe quantum dots on electrodes with different morphology

We have studied the optical absorption of CdSe quantum dots (QDs) adsorbed on inverse opal TiO2 (IO-TiO2) and nanoparticulate TiO2 (NP-TiO2) electrodes using photoacoustic (PA) measurements. The CdSe QDs were grown directly on IO-TiO2 and NP-TiO2 electrodes by a successive ionic layer adsorption and reaction (SILAR) method with different numbers of cycles. The average diameter of the QDs was estimated by applying an effective mass approximation to the PA spectra. The increasing size of the QDs with increasing number of cycles was confirmed by a redshift in the optical absorption spectrum. The average diameter of the CdSe QDs on the IO-TiO2 electrodes was similar to that on the NP-TiO2 ones, indicating that growth is independent of morphology. However, there were more CdSe QDs on the NP-TiO2 electrodes than on the IO-TiO2 ones, indicating that there were different amounts of active sites on each type of electrode. In addition, the Urbach parameter of the exponential optical absorption tail was also estimated from the PA spectrum. The Urbach parameter of CdSe QDs on IO-TiO2 electrodes was higher than that on NP-TiO2 ones, indicating that CdSe QDs on IO-TiO2 electrodes are more disordered states than those on NP-TiO2 electrodes. The Urbach parameter decreases in both cases with the increase of SILAR cycles, and it tended to move toward a constant value.

A detailed examination of the growth of CdSe thin films through structural and optical characterization

Different thickness CdSe thin films were grown on glass substrates by physical vapor deposition and characterized by optical and structural investigations. Urbach energy related to the width of the optical absorption tail decreases from 430meV for a film thickness of 50nm to 200meV for 450nm. The film thickness dependent grain sizes were estimated by using effective mass model under quantum size effect from the shift of around 500meV in the asymptotic absorption edge. The X-ray diffraction (XRD) pattern is consistent with CdSe hexagonal crystal structure which indicates crystal growth mode along c axis. XRD peaks broaden and shift depending on film thicknesses which are presumably due to strain and size effect. We observe both blue and red shift depending on thickness in Longitudinal Optical phonon frequency in Raman spectra with respect to that of the source CdSe powder which could also be due to strain on thin films and/or finite crystallite size. In this work we combine the results of optical absorption, Raman and XRD spectroscopies to study the evolution of grain size, strain and structural disorder depending on film thickness.

Modulation of residual stress in diamond like carbon films with incorporation of nanocrystalline gold

Residual stress modulation in the diamond-like carbon coatings with incorporation of gold nanoparticles was studied critically. The films were deposited on glass and Si (1 0 0) substrates by using capacitatively coupled plasma chemical vapor deposition. Stresses in the films were determined from the broadening of the optical absorption tail and were found to decrease from 2.3 GPa to 0.48 GPa with increasing gold content (2–7 at.% Au) in the DLC matrix. Gold incorporation also made the films harder than the corresponding DLC coatings. Modulation of stress with nanocrystalline gold content in the DLC matrix was related to the relative amount of sp 2/sp 3 content in the DLC films.

A quantitative characterization of the optical absorption spectrum associated with hydrogenated amorphous silicon

We propose a quantitative means of characterizing the optical absorption spectrum associated with an amorphous semiconductor. In particular, for a representative hydrogenated amorphous silicon optical absorption experimental data set, through a series of least-squares linear fits of an exponential function to this experimental data set, taken over a number of optical absorption ranges, we determine how the breadth of the optical absorption tail varies along the optical absorption spectrum of hydrogenated amorphous silicon. We find that the quantitative variations in the breadth of the optical absorption tail that are found provide for a clear delineation between the different regions of the optical absorption spectrum of hydrogenated amorphous silicon. We complete this analysis by theoretically determining the form of the optical absorption spectrum using a recently developed empirical model for the density of states functions corresponding to hydrogenated amorphous silicon, this analysis providing a theoretical basis for the interpretation of our results.

Synthesis of N-doped titanium oxide by hydrothermal treatment

N-doped TiO2 nanoparticles have been prepared by continuous or batch treatment of the colloidal TiO2 nanoparticles under hydrothermal conditions with formamide, nitric acid, ammonia or triethylamine. The optical absorption tail of the obtained photocatalysts near the band gap extended to the visible region around λ = 500 nm. The efficiency of nitrogen doping was strongly dependent on the kind of nitrogen compounds, treatment method as well as treatment temperature. The most effective N-doping could be accomplished by the batch treatment with a small amount of triethylamine, which showed the efficient photocatalytic activity for both the reduction of Ag+ ions and the degradation of methylene blue under visible light irradiation.

Optical constants of silicone-like (Si:O x:C y:H z) thin films deposited on quartz using hexamethyldisiloxane in a remote RF hollow cathode discharge plasma

Deposition of amorphous silicone-like (Si:O x :C y :H z ) thin films in a remote RF hollow cathode discharge plasma using hexamethyldisoloxane as monomer and Ar as feed gas has been investigated for films optical constants and plasma diagnostic as a function of RF power (100–300 W) and precursor flow rate (1–10 sccm). Plasma diagnostic has been performed using Optical Emission Spectroscopy (OES). The optical constants (refractive index, extinction coefficient and dielectric constant) have been obtained by reflection/transmission measurements in the range 300–700 nm. It is found that the refractive index increases from 1.92 to 1.97 with increasing power from 100 to 300 W, and from 1.70 to 1.92 with increasing precursor flow rate from 1 to 10 sccm. The optical energy band gap E g and the optical-absorption tail Δ E have been estimated from optical absorption spectra, it is found that E g decreases from 3.28 to 3.14 eV with power increase from 100 to 300 W, and from 3.54 to 3.28 eV with precursor flow rate increase from 1 to 10 sccm. Δ E is found to increase with applied RF power and precursor flow rate increase. The dependence of optical constants on deposition parameters has been correlated to plasma OES.

Influence of growth temperature on order within silicon films grown by ultrahigh-vacuum evaporation on silica

We study the role that the growth temperature plays in determining the amount of order present within silicon films deposited on fused silica substrates through ultrahigh-vacuum evaporation at growth temperatures ranging from 98 to 572°C. Through measurements of the Raman and optical absorption spectra, we quantitatively determine how the growth temperature influences the order present within 11 such films. We employ three disparate measures of order for the purposes of this study: the breadth of the transverse-optic phonon Raman peak, this being related to the amount of short-range order present; the area under the transverse-acoustic Raman peak divided by the area under the corresponding transverse-optic peak, this being related to the amount of intermediate-range order present; and the breadth of the optical absorption tail, which is a general measure of the overall amount of order present. All three measures of order indicate a dramatic increase in the amount of order present for growth temperatures above 400°C.

Structural and optical properties of amorphous hydrogenated silicon carbonitride films produced by PECVD

Amorphous hydrogenated silicon carbonitride thin films (a-Si:C:N:H), deposited by plasma enhanced chemical vapour deposition (PECVD) using hexamethyldisilazane (HMDSN) as monomer and Ar as feed gas, have been investigated for their structural and optical properties as a function of the deposition RF plasma power, in the range of 100–300 W. The films have been analysed by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), UV–vis–NIR spectrophotometry and atomic force microscopy (AFM). From the analysis of the FT-IR spectra it results that the films become more amorphous and inorganic as RF plasma power increases. The incorporation of oxygen in the deposited layers, mainly due to the atmospheric attack, has been evaluated by XPS and FT-IR spectroscopy. Reflectance/transmittance spectra, acquired in the range of 200–2500 nm, allow to descrive the film absorption edge for interband transitions. A relationship between the optical energy band gap, deduced from the absorption coefficient curve, and the deposition RF plasma power has been investigated. The reduction of the optical energy gap from 3.85 to 3.69 eV and the broadening of the optical absorption tail with RF plasma power increasing from 100 to 300 W are ascribed to the growth of structural disorder, while the increase of the refractive index, evaluated at 630 nm, is attributed to a slight densification of the film. The AFM analysis confirms the amorphous character of the films and shows how the deposited layers become rougher when RF plasma power increases. The wettability of the film has been studied and related to the chemical composition and to the morphology of the deposited layers.

Optical and structural properties of siliconlike films prepared by plasma-enhanced chemical-vapor deposition

Amorphous siliconlike thin films (Si:Ox:Cy:Hz), deposited by plasma-enhanced chemical-vapor deposition using hexamethyldisiloxane as monomer and Ar as feed gas, have been investigated for their optical and structural properties as a function of the deposition power, in the range of 100–400 W. The films have been analyzed by Fourier transform infrared spectroscopy (FTIRS), UV-VIS-NIR spectrophotometry, and atomic force Microscopy (AFM). From the analysis of FTIR spectra it results that films assume a marked inorganic character as the power increases. Indeed, at high power, Si–O–Si groups prevail over Si(CH3)x groups, so that the film chemistry approaches the SiO2 one. Reflectance∕transmittance spectra, acquired in the range of 200–2500 nm, allow us to describe the film absorption edge for interband transitions. The relationship between the optical energy band gap, deduced from the absorption coefficient curve, and the deposition power has been investigated. The reduction of the optical energy gap from 3.86 to 3.61 eV and the broadening of the optical-absorption tail with the increase of power from 100 to 400 W are ascribed to the growth of thermal and structural disorders. Moreover, the refractive index has been evaluated and related to the film morphology. The AFM analysis confirms the amorphous character of the films and shows how the deposited layers become flatter and more compact when power increases. We consider the densification of the film responsible for the growth of the refractive index from 1.90 to 1.97 in the power range 100–400 W.

Relationship between the optical gap and the optical-absorption tail breadth in amorphous GaAs

We study the relationship between the optical gap and the optical-absorption tail breadth for the case of amorphous gallium arsenide (a-GaAs). In particular, we analyze the optical-absorption spectra corresponding to some recently prepared a-GaAs samples. The optical gap and the optical-absorption tail breadth corresponding to each sample is determined. Plotting the optical gap as a function of the corresponding optical-absorption tail breadth, we note that a trend, similar to that found for the cases of the hydrogenated amorphous silicon and hydrogenated amorphous germanium, is also found for the case of a-GaAs. The impact of alloying on the optical-absorption spectrum associated with a-GaAs is also briefly examined.

The influence of crystal imperfections on the shape of exciton emission spectrum in ZnO single crystals

A distortion of exciton emission spectrum, namely, relative decrease of free exciton band intensity with respect to that of its phonon replicas in undoped as-grown ZnO single crystals has been shown to result from enhanced reabsorption of shorter wavelength emission in the crystal due to formation of optical absorption tail. Some crystal imperfections, in all probability, dislocations decorated with shallow donors are supposed to be responsible for the effect.

Hydrogen and disorder in diamond-like carbon

Diamond-like carbon is a system of rather high disorder as it has a wide optical absorption tail and a high density of paramagnetic defects. The defect density remains high even in DLCs containing 30-60% hydrogen, so hydrogen does not appear to passivate defects well unlike in a-Si:H. To investigate the role of hydrogen on the disorder in DLCs we have investigated the effect of low concentrations of hydrogen on the disorder in ta-C, by introducing 10(-6)-10(-3) mbar hydrogen into the deposition of ta-C by filtered cathodic vacuum are (FCVA), which corresponds to 0.1-15 at.% hydrogen in the films. Higher pressures of hydrogen reduces the ionisation leading to sp(2) bonding, and ultimately the thermalisation of the plasma leads to nanotubes and fullerenes. The deposited ta-C:H films were investigated by electron energy loss spectroscopy (EELS), Raman spectroscopy, optical measurements, electronic transport and N-15 resonant nuclear reaction analysis. Plasma characterisation with a retarding field analyser showed that the ion current density remains nearly unchanged in the pressure range used to deposit the films. Raman measurements indicate the onset of clustering of sp(2) sites when the hydrogen pressure exceeds 2 x 10(-4) mbar. We find that small amounts of hydrogen increase the optical gap up to 2X10(-6) mbar hydrogen pressure, and then the band gap decreases continuously. The absorption tail sharpens by the addition of hydrogen, as measured by photothermal deflection spectroscopy (PDS) and thus confirms the Raman measurements that suggest that the order in the material increases with increasing hydrogen content.

A Raman spectroscopic study on the microscopic origin of the photoinduced fluidity effect

We report on a sub-band gap light scattering (Raman) investigation for a-As 2S 3 fibers subjected to external elongation stress in order to elucidate the photoinduced fluidity effect on a microscopic scale. Changes in the short-range order caused by the presence of the illuminating light have been detected. On the other hand, subtle modifications are revealed in the intermediate range order, manifested in the increase of the magnitude and in the modification of the frequency dependence for the depolarization ratio – in the low-frequency part of the spectrum – as a function of the applied stress. The “buckling model” proposed to account for the optical absorption tails in chalcogenide glasses seems quite relevant for explaining certain aspects of the experimental data.

Optical Absorption and Urbach Tails in Layered Semiconductor Tl0.999GaPr0.001Se2 and TlGaSe2

Optical Absorption and Urbach Tails in Layered Semiconductor Tl0.999GaPr0.001Se2 and TlGaSe2