Exponential Absorption Edge Research Articles

Exponential optical absorption edge in PbS quantum dot-ligand systems on single crystal rutile-TiO2 revealed by photoacoustic and absorbance spectroscopies

The photovoltaic properties of quantum dot (QD) sensitized solar cells (QDSCs) depend significantly on the surface modification applied to the QDs and on the nanostructured interface between the QDs and the electrode surface. In the development of QDSCs with spatially ordered QD arrays, linking molecular ligands with the QDs (QD-ligands) can lead to the realization of novel QDSCs. The ligand shell around the QDs mediates the electron and energy transfer processes that underpin their use in QDSC applications. The dependence of the photovoltaic properties on the interparticle distance (QD spacing) can also be evaluated by applying different sizes of molecular ligands. The present study focuses on specific attention to the exponential optical absorption edge (often termed Urbach tail) in PbS QD-ligand systems with different QD spacing adsorbed on rutile-TiO2 (R-TiO2) substrates with different crystal orientations. It is essential to accurately characterize QD-ligands on electrode surfaces with different crystal orientations, not only for scientific studies, but to further optimize the growth conditions and processes in order to design and fabricate advanced QDSCs. Photoacoustic (PA) and conventional absorbance (Abs) spectroscopies were applied to determine the optical absorption and nonradiative relaxation properties. There is a discrepancy between the PA and Abs spectra especially in the Urbach tail region. As the Urbach tail states are related to the absorbed photon energy lost in the form of heat generated by nonradiative relaxation, therefore characterization of the Urbach tail is important and essential for QDSC applications. Characterization of the Urbach tail and the heat generated by nonradiative relaxation of PbS QD-ligand systems by combined PA and Abs spectroscopies showed that the characteristics depend strongly on the crystal orientation of the R-TiO2 substrate, the QD spacing, and the free energy change.

Electron energy loss spectra from silica glass optical fibers

AbstractTo investigate the possible structural differences between silica glass fibers and bulk silica glasses, electron energy loss spectroscopy (EELS) has been used to study the short‐range and medium‐range structures of both forms of silica glasses. The short‐range structure of silica glass, such as the coordination and symmetry, was investigated by the energy loss near edge structure (ELNES) of Si L2,3‐edges. The ordering structure in the medium‐range was analyzed by the exponential optical absorption edge also known as the Urbach edge of the glasses. The optical absorption data were obtained from the low energy loss spectrum of EELS through Kramers‐Kronig analysis. The results show that silica fiber has the same short‐range structure as the bulk specimen, but is significantly more disordered than the bulk glasses.

Determination of absorption coefficients and Urbach tail depth of ZnO below the bandgap with two-photon photoluminescence.

In this article, we demonstrate a novel approach to determine the absorption coefficient of ZnO below the bandgap via measuring the self-absorption (SA) effect on the two-photon luminescence (TPL) spectrum of the ZnO bulk crystal rod at cryogenic temperature. Under a geometric configuration of side-excitation and front-detection, the intensities of several major spectral components of TPL spectra of ZnO can be decisively tuned by precisely varying the transmitting distance of luminescence signal, so that the absorption coefficients at different wavelengths can be determined on the basis of Beer-Lambert law. Furthermore, the peak position of donor bound exciton luminescence exhibits a unique redshift tendency with increasing the transmitting distance. Starting from the product of Lorentzian lineshape function and exponential absorption edge of Urbach tail, an analytical formula is derived to quantitatively interpret the experimental redshift characteristic with the transmitting distance. The energy depth of Urbach tail of the studied ZnO crystal is deduced to be ∼13.3 meV. In principle, this new approach can be used to determine absorption coefficient of any luminescent solids as long as the SA effect happens.

Band-fluctuations model for the fundamental absorption of crystalline and amorphous semiconductors: a dimensionless joint density of states analysis

We develop a band-fluctuations model which describes the absorption coefficient in the fundamental absorption region for direct and indirect electronic transitions in disordered semiconductor materials. The model accurately describes both the Urbach tail and absorption edge regions observed in such materials near the mobility edge in a single equation with only three fitting parameters. An asymptotic analysis leads to the universally observed exponential tail below the bandgap energy and to the absorption edge model at zero Kelvin above it, for either direct or indirect electronic transitions. The latter feature allows the discrimination between the absorption edge and absorption tails, thus yielding more accurate bandgap values when fitting optical absorption data. We examine the general character of the model using a dimensionless joint density of states formalism with a quantitative analysis of a large amount of optical absorption data. Both heavily doped p-type GaAs and nano-crystalline Ga1-xMnxN, as examples for direct bandgap materials, as well as amorphous Si:Hx, SiC:Hx and SiNx, are modeled successfully with this approach. We contrast our model with previously reported empirical models, showing in our case a suitable absorption coefficient shape capable of describing various distinct materials while also maintaining the universality of the exponential absorption tail and absorption edge.

A possible physical mechanism for the formation of the exponential fundamental absorption edge in indirect gap semiconductors

A possible physical mechanism for the formation of the exponential fundamental absorption edge in indirect gap semiconductors

Crystal Growth, Exponential Optical Absorption Edge, and Ground State Energy Level of PbS Quantum Dots Adsorbed on the (001), (110), and (111) Surfaces of Rutile-TiO2

It is important to investigate the dependencies of the optical absorption and the ground state energy level on the size of the semiconductor quantum dots (QDs) on fully studied single crystal TiO2 surfaces. The present study focuses on the systems comprising PbS QDs on (001), (110), and (111) surfaces of single crystal rutile-TiO2. By the optical absorption characterization, the average diameter of PbS QDs on a (001) surface is independent of the number of adsorption cycles, although those on (110) and (111) surfaces increase with the number of cycles. The rate of adsorption of PbS QDs on a (001) surface is higher than those grown on (110) and (111) surfaces. The results suggest that the crystal growth is caused by the difference of the surface energy of the substrate. The exponential optical absorption edge suggests that the structural disorder of PbS QDs on (001) and (110) surfaces increases as the number of adsorption cycles increases. On the other hand, that on a (111) surface decreases as the number ...

Temperature studies of optical parameters of (Ag3AsS3)0.6(As2S3)0.4 thin films prepared by rapid thermal evaporation and pulse laser deposition

(Ag3AsS3)0.6(As2S3)0.4 thin films were deposited using rapid thermal evaporation (RTE) and pulse laser deposition (PLD) techniques. Ag-enriched micrometre-sized cones (RTE) and bubbles (PLD) were observed on the thin film surface. Optical transmission spectra of the thin films were studied in the temperature range 77–300K. The Urbach behaviour of the optical absorption edge in the thin films due to strong electron–phonon interaction was observed, the main parameters of the Urbach absorption edge were determined. Temperature dependences of the energy position of the exponential absorption edge and the Urbach energy are well described in the Einstein model. Dispersion and temperature dependences of refractive indices were analysed; a non-linear increase of the refractive indices with temperature was revealed. Disordering processes in the thin films were studied and compared with bulk composites, the differences between the thin films prepared by RTE and PLD were analysed.

Urbach absorption edge in epitaxial erbium-doped silicon

We investigate the dependencies of the photocurrent in Si:Er p-n junctions on the energy of the incident photons. The exponential absorption edge (Urbach edge) just below fundamental edge of silicon was observed in the absorption spectra of epitaxial Si:Er layers grown at 400–600 C. It is shown that the introduction of erbium significantly enhances the structural disorder in the silicon crystal which was estimated from the slope of the Urbach edge. We discuss the possible nature of the structural disorder in Si:Er and a new mechanism of erbium excitation, which does not require the presence of deep levels in the band gap of silicon.

Optical properties and band structure of a layered Tl2S crystal

The optical absorption spectra of a layered Tl2S crystal have been studied at T = 293 K. In has been found that the fundamental absorption edge has an exponential shape in all samples under study. The ab initio calculations of the energy band structure of Tl2S have been performed in terms of the density functional theory for the first time. Based on the complementary analysis of the results of the optical measurements and calculation of the energy band structure of the material, the model of formation of the exponential fundamental absorption edge of the layered semiconductor Tl2S has been proposed.

Peculiarities of the Ni2+ ions in the crystal lattice of Bi12SiO20:Ni

Absorption measurements are taken over a wide spectral range which includes the visible, near and far IR region (500–22,272cm−1). The exponential absorption edge parameters σ(T), σ0, Wd and g are calculated for undoped and Ni doped Bi12SiO20 at room temperature. The dopant nickel ion is double-charged and occupies the tetrahedral sites in the crystal lattice of doped sillenite. The energy level structure of the Ni2+ ion in Bi12SiO20:Ni (BSO:Ni) is also presented. The crystal field parameter Dq and the Racah parameters B and C are calculated. It is established that the Jahn–Teller effect displays very strong influence in a distored tetrahedral Ni2+ complex. The two-photon absorption of BSO:Ni is studied in far IR region.

Influence of ion implantation on optical parameters of Cu<sub>6</sub>PS<sub>5</sub>I thin films

The ion implantation of Cu 6 PS 5 I thin films by sulphur and phosphorous is carried out as well as the structural studies by methods of scanning electron microscopy and energy-dispersive X-ray spectroscopy are performed. Transmission spectra of thin films and influence of ion implantation on spectra and parameters of exponential absorption edge are investigated. Refractive indeces dispersion of implanted thin films is studied as well as its description by optical-refractometric relation is carried out. Dependences of the optical pseudogap, Urbach energy and refractive indeces in implanted films on fluence values are investigated and analyzed.

Optical and photoelectric properties of α-Dy2S3 and α-Dy2S3〈Pb〉 films

A process is described for the growth of thin crystalline α-Dy2S3 films by thermal evaporation from separate dysprosium and sulfur sources. The films were doped with Pb, and their reflection and transmission spectra were measured at room temperature and photon energies in the range (0.3−5.2) × 10−19 J. The α-Dy2S3 films were shown to have an exponential absorption edge. The photoconductivity of the doped films was measured at photon energies in the range (0.3−5.2) × 10−19 J and temperatures from 115 to 400 K.

Spectrometric and ellipsometric studies of (1 − x)TiO2 · xLn2O3 (Ln = Nd, Sm, Gd, Er, Yb) thin films

Spectrometric and ellipsometric studies of (1−x)TiO2·xLn2O3 (Ln=Nd,Sm,Gd,Er,Yb; x=0.33,0.5) thin films at room temperature were performed. The obtained dispersion dependences of refractive indices are successfully described by the optical-refractometric relation. The dependence of optical pseudogap and refractive indices on composition and molar mass of the films is investigated. The influence of compositional disordering on the energy width of the exponential absorption edge is studied.

Optical and photoelectric properties of pure and cadmium-and lead-doped neodymium sesquisulfide thin films

A technique for preparing γ-Nd2S3 crystalline thin films through discrete vacuum thermal evaporation of a presynthesized bulk material is developed. The films deposited are doped with cadmium and lead. The reflectance and transmittance spectra of the films are measured in the photon energy range 0.2–3.0 eV at a temperature of 300 K. The frequency analysis of the absorption coefficient demonstrates that the γ-Nd2S3 films are characterized by an exponential absorption edge. The photoconductivity spectra and temperature dependences of the photoconductivity for the γ-Nd2S3 films doped with cadmium and lead are measured in the photon energy range 0.2–3.3 eV at temperatures varying from 115 to 380 K. The experimental data obtained are interpreted under the assumption that the acceptor levels formed by vacancies in the cation sublattice and compensated for by cadmium and lead donor dopants play a crucial role in the photoconduction. The ionization energy at the lead donor level is determined.

Optical study of disorder and defects in hydrogenated amorphous silicon carbon alloys

We have studied the defect density and disorder in hydrogenated amorphous silicon carbon alloys produced by rf glow discharge of silane-methane-hydrogen mixtures, by combining spectroscopic ellipsometry and photothermal deflection spectroscopy measurements. Increasing the methane flow rate leads to a widening of the optical gap and to an increase of the apparent disorder, deduced from the standard analysis of the exponential absorption edge; the so-called Urbach energy. Interestingly, the subgap absorption decreases with increasing methane flow rate. This points towards a lower density of defects with increasing carbon content and is in contrast with the increased disorder. This apparent contradiction results from the presence of three absorption bands within the gap of this material, as reported by [Ivashchenko et al., J. Phys.: Condens. Matter 14, 1799 (2002)], and which make unreliable the standard analysis of the disorder in silicon carbon alloys.

Phase transitions and optical absorption edge in CuInP2Se6 layered ferrielectrics

Temperature studies of optical absorption spectra in the range of the fundamental absorption edge for layered CuInP 2 Se 6 single crystals obtained by the technique combining recrystallization and vapor-phase transport, are reported. The temperature behavior of the absorption-edge spectra is the evidence for the essential changes of the dynamical disordering of CuInP 2 Se 6 crystal structure at second-order (T 0 ≃249K) and first-order (T c ≃236K) phase transitions. The present anomalous temperature hysteresis and the glass-like behavior of the exponential absorption edge in the intermediate phase resemble those being observed in the incommensurate phases of ferroelectrics.

Compositional disordering and some structural and optical parameters of Cu 6P(S 1− xSe x) 5Br mixed crystals

A series of Cu 6P(S 1− x Se x ) 5Br solid solutions was studied at room temperature by X-ray diffraction and optical absorption measurements in the whole compositional range (0≤ x≤1). The optical pseudogap, the absorption edge energy width, the cubic lattice parameter and calculated density of these mixed crystals were all found to change non-linearly with the composition x. These non-linear compositional dependencies were attributed to the effect of compositional disordering on the energy position and shape of exponential fundamental optical absorption edge in Cu 6P(S 1− x Se x ) 5Br.

Influence of compositional disorder on optical absorption processes in Cu6P(S1−xSex)5I crystals

The fundamental absorption edge of Cu6P(S0.6Se0.4)5I and Cu6P(S0.4Se0.6)5I crystals was studied in the 77–320 K temperature range where no phase transitions were revealed by optical techniques. The temperature behavior of the exponential Urbach absorption edge in Cu6P(S0.6Se0.4)5I and Cu6P(S0.4Se0.6)5I crystals was analyzed. The effect of compositional disorder upon the Urbach edge parameters, parameters of exciton-phonon interaction, and lattice parameters in Cu6P(S1−xSex)5I, 0 ≤ x ≤ 1, crystals was studied. The effect of various types of disordering upon the optical absorption processes in Cu6P(S1−xSex)5I crystals is discussed.

Excitation mechanisms and structure-related Er 3+ emission in amorphous and nanocrystalline GaN films

We report on Er 3+ luminescence at room temperature in the visible and infrared regions in amorphous GaN:Er films prepared by DC magnetron co-sputtering. The intensity of the Er 3+ luminescence at 1535 nm corresponding to the 4 I 13/2→ 4 I 15/2 transition increased after annealing at 750°C. During annealing GaN crystallites form in the amorphous matrix. The crystallite diameters are between 4 and 7 nm as analyzed by high-resolution transmission electron microscopy (HRTEM). A shallow exponential absorption edge extends three orders of magnitude in absorption coefficient over the spectral range from 0.5 to 3.5 eV. On this exponential absorption are superimposed the resonant absorption bands of Er 3+ ions.

Investigation of the nature of the intermediate quasipolar phase in CuCrP<sub>2</sub>S<sub>6</sub> crystals

Temperature behaviour of dielectric permeability and optical absorption edge in the range of phase transitions (PT) for CuCrP 2 S 6 crystals is studied. The dielectric permeability temperature dependence exhibits a knee at the second-order PT from paraelectric to intermediate phase (Т c 1 ~190K)., a step at the first-order PT from the intermediate to antiferroelectric phase (Т c 2 ~150K) and temperature hysteresis in the whole intermediate phase range. The absorption edge is shown to be described by Urbach rule with different convergence point coordinates in antiferroelectric and paraelectric phases. In the intermediate phase there is no convergence point and parallel shift of the exponential absorption edge is observed. The influence of In?Cr cationic substitution on the Urbach absorption edge is analyzed. The specific features of the temperature behaviour of the dielectric permeability and the absorption edge are treated assuming the intermediate phase in CuCrP 2 S 6 crystals to be an incommensurate.