Photoconductivity in crystalline phthalocyanines

The wavelength, temperature, time and intensity dependence of photocurrent of metal-free phthalocyanine (H₂Pc) and copper phthalocyanine (CuPc) single crystals were investigated. The thermal activation energies in the dark are 0·5 and 0·6 eV for H₂Pc and CuPc respectively and the corresponding photo-thermal activation energies are 0·3 and 0·2 eV. An energy level scheme for single crystals of H₂Pc and CuPc is proposed which consists of two trapping levels and five narrow optically active valence bands. In H₂Pc (CuPc), one trapping level at 0·5 eV (0·6 eV) above the valence band edge to which the charge carriers are thermally excited in the dark; and the other trapping level is at 0·3 eV (0·2 eV) below the conduction band edge where all the optical transitions terminate. In H₂Pc(CuPc), the forbidden gap is 1·44 eV (1·34 eV) wide; the five valence bands are at the band edge, and 0·09 (0·22), 0·42 (0·63), 0·69 (0·90), 1·32 (2·17) eV below the band edge.     

Conduction mechanism in anthracene as a function of temperature

Conduction mechanism in anthracene single crystal grown by Bridgman method was carried out. The investigations consisted of dark- and photo-current variation with respect to (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·5 to 2·5 kV/cm and the temperature range was between 300 K and 450 K. Photo and dark current variations with temperature indicate, based on activation energy determination, that a band model can be applied to the conduction process. The band gap is calculated to be 1·6 eV. The band model consists of a recombination centre 0·37 eV above the valence band edge and a trap level 0·55 eV below the conduction band edge to which electrons are first thermo-optically excited and then they are thermally excited into the conduction band.     

Photoconductivity in thin films of phthalocyanine

The photocurrent and electrolyte electromodulation (EEM) spectra of thin films of metal-free phthalocyanine (H₂Pc) and of copper phthalocyanine (CuPc) were investigated. The modulation spectra yielded three distinct features around 1·61, 2·30 and 2·93 eV for H₂Pc and around 1·63, 2·04 and 3·20 eV for CuPc. The spectral dependence maxima of photoconductivity correspond to the modulation spectra. These features are interpreted to indicate transitions at critical points, i.e. the existence of transitions between three valence bands, since Pc’s are p-type, and the lowest conduction band in Pc’s.     

The optical properties of CuInS2 thin films

The optical absorption in flash-evaporated CuInS₂ thin films was studied in the photon energy range from 0.5 to about 4.2 eV. CuInS₂ was found to be a direct gap semiconductor with a gap energy of 1.524±0.005 eV at room temperature. The ground state energy of the free exciton was found to be about 8 meV. An indirect allowed transition was observed at 1.565±0.005 eV and was ascribed to an optical transition from the valence band maxima at the boundary of the Brillouin zone to the lowest conduction band minimum at the zone centre. Three further optical transitions which were probably due to the copper d states in the valence band were found at energies well above the fundamental edge.     

Ag2СdSnS4 single crystals as promising materials for optoelectronic

Single crystals of the quaternary single crystals Ag₂CdSnS₄ were grown for the first time using the horizontal gradient freeze technique. Optical spectral and photoelectric properties of obtained crystals were investigated. The band gap energy at 77 K according to the photoconductivity spectra is 1.94 eV. The energy levels of the major donor centers in the band gap were determined. The role of intrinsic defects in the observed dependences is analyzed. The energy levels of the major donor centers in the band gap were determined. A small photoconductivity maximum at low temperature is observed at wavelength λ_m = 640 nm (hν ∼ 1.94 eV); situated in the fundamental absorption band, which unambiguously corresponds to the intrinsic origin of photoconductivity. The increase of the extrinsic photoconductivity with the maximum at λ_m ∼ 800 nm with temperature leads to its domination above 240 K. The observed peculiarity can be explained by the photoexcitation of electrons from the valence band to the donor centers which are empty at high temperatures and with further thermal excitation to the conduction band.     

The optical properties of AgInTe2 thin films

The optical absorption in electron-beam-evaporated AgInTe₂ thin films was studied in the energy range 0.5–2 eV. AgInTe₂ was found to be a direct gap semiconductor with a room temperature gap of 1.03±0.01 eV. Another direct transition observed at 1.04±0.01 eV was ascribed to an optical transition from the crystal-field-split valence band to the conduction band minimum. A third direct allowed transition from the spin-orbit-split valence band to the conduction band was identified at 1.77±0.03 eV. An estimate of the p-d hybridization of the uppermost valence bands yields a value of about 15%.     

Development of nanostructured CdS sensor for H2S recognition: structural and physical characterization

In the present work, we report on the performance of a nanostructured CdS gas sensor. The sensor was fabricated using spin coating technique on glass substrate. The CdS sensor was characterized for their, structural microstructural as well as optoelectronic and H₂S response was studied. The XRD analysis showed formation of nanocrystalline CdS. Morphological analysis using SEM revealed nanostructured morphology with average grain size in the range of 40–50nm. Optical investigations showed a high absorption coefficient (10⁴ cm⁻¹) with a direct band gap of 2.54 eV. Electrical transport studies revealed films shows n-type conduction mechanism with room temperature dc electrical conductivity 10⁻⁶ (Ω cm)⁻¹. The CdS sensors showed the maximum response of 13.2% upon exposure to 100 ppm H₂S at operating temperature 100 °C.     

Temperature effects on the optoelectronic properties of AgIn5S8 thin films

Polycrystalline AgIn₅S₈ thin films are obtained by the thermal evaporation of AgIn₅S₈ crystals onto ultrasonically cleaned glass substrates under a pressure of ~ 1.3 × 10⁻³ Pa. The temperature dependence of the optical band gap and photoconductivity of these films was studied in the temperature regions of 300-450 K and 40-300 K, respectively. The heat treatment effect at annealing temperatures of 350, 450 and 550 K on the temperature dependent photoconductivity is also investigated. The absorption coefficient, which was studied in the incidence photon energy range of 1.65-2.55 eV, increased with increasing temperature. Consistently, the absorption edge shifts to lower energy values as temperature increases. The fundamental absorption edge which corresponds to a direct allowed transition energy band gap of 1.78 eV exhibited a temperature coefficient of −3.56 × 10⁻⁴ eV/K. The 0 K energy band gap is estimated as 1.89 eV. AgIn₅S₈ films are observed to be photoconductive. The highest and most stable temperature invariant photocurrent was obtained at an annealing temperature of 550 K. The photoconductivity kinetics was attributed to the structural modifications caused by annealing and due to the trapping-recombination centers' exchange.     

Amino acid 2-aminopropanoic CH3CH(NH2)COOH crystals: materials for photo- and acoustoinduced optoelectronic applications

Photo-induced treatment of l-alanine single crystals grown by slow evaporation method at an ambient temperature was performed using a 25 ps Nd:YAG pulsed laser in the presence of an external acoustic filed. The changes of the absorption were studied for the wavelength 265 nm near the energy band gap edge at acoustical power density varying within 4–6 W/cm². The observed absorption changes indicate that the external optical electric field strengths and acoustical power densities may be efficient parameters for the characterization of photo-optical and acousto-optical treatment of the samples. From the X-ray diffraction data we have optimized the atomic positions assuming that force on the atoms is around 1 mRy/au. These are used to calculate the electronic structure and the chemical bonding for the amino acid l-alanine single crystals. The calculated electronic band structure and densities of states confirms the experimental results that this compound possesses a relatively large energy band gap. The upper valence band has its maximum at the Z point of the Brillouin zone while the conduction band minimum is located at Γ point in the zone center, resulting in an indirect energy band gap. The electronic energy gap is equal to 4.19 eV within a framework of the used local density approximation and 4.54 eV with the Engel–Vosko generalized gradient approximation as the exchange correlation potential. This is in an agreement with our experimentally measured energy band gap ~4.67 eV. The existence of O-p character in the upper valence band has a significant consequence for the optical band gap. From our calculated electron charge density distribution, we obtain a space electron charge density distribution in the average unit cell of the crystal. The chemical bonding features of l-alanine amino acid were analyzed.     

Investigation of electrical conduction in polyvinyl formal

Current-voltage (I–V) characteristics of pure polyvinyl formal (PVF) were investigated at different fields, range 5–100 kV/cm, as a function of temperature, range 313–363 K. It was observed that while at low fields (up to 25 kV/cm), the conduction was Ohm’s law-dependent at high fields (beyond 25 kV/cm), the conduction was Poole-Frenkel (P-F) mechanism-dependent. An attempt was made to identify the nature of the current by comparing its observed dependence on temperature, electric field and electrode materials with their respective characteristic features of the existing theories of electrical conduction. The current showed a strong dependence on temperature. To identify the possible mechanism of conduction, current versus square root of field characteristics were drawn with aluminium, silver, copper and gold as upper electrodes and Al as the lower electrodes. The observed characteristic suggested that the charge carriers were generated by the field-assisted lowering of coulombic barriers at the traps, and were subsequently conducted through the bulk of the material by a hopping process between the localized states by a Jonscher-Ansari-modified P-F mechanism. The calculated value of the modified P-F barrier was ⋍ 1·94×10⁻¹⁹ J (1·21 eV).     

Role of lanthanum in thermoluminescence properties of La<Subscript>2<Emphasis Type="Italic">x</Emphasis></Subscript>Lu<Subscript>2(1−<Emphasis Type="Italic">x</Emphasis>)</Subscript>SiO<Subscript>5</Subscript>:Ce crystals

The influence of lanthanum content on thermally stimulated luminescence properties of La_2xLu_2(1?x)SiO₅:Ce (x = 0, 0.08, 0.18 and 1.50 at.%) crystals was investigated. Trapping parameters such as electron trap depth E_t and electron traps content n ₀ were fitted with general order kinetic function. According to the results of the VUV transmittance spectra and band gap calculation which based on the density functional theory with the generalized gradient approximation of Perdew–Burke–Ernzerhof (GGA–PBE), band structure of the crystals and recombination mechanisms of released electrons were further studied. Results showed that there were mainly two kinds of electron traps, namely shallow (E_t ≈ 0.2 eV) and deep (E_t ≈ 1.0 eV) traps. With the increasing of La content, the concentration of deep traps was obviously depressed, and the depth of deep traps decreased at the same time, which was explained by suggesting the downward shift of the bottom of conduction band.     

Growth and properties of sodium tetraborate decahydrate single crystals

Sodium tetraborate decahydrate (Na₂B₄O₇·10H₂O) single crystals were grown from aqueous solution by slow solvent evaporation method and its functional groups have been confirmed by FTIR. Grown crystals were found to be hard in nature (n = 1.8). From optical studies band gap of the material was found to be 4.1 eV and PL emission peak was found at 340 nm which can be due to annihilation self trapped excitons present in borate crystals. Dielectric constant was found to increase with temperature up to 70 °C. Conductivity obeys Jonscher's power law as seen at different temperatures. Activation energy of the crystal was found to be 2.05 eV.     

Electrical properties of Se1−xTex crystals

Single crystals of Se_1?xTe_x (with x varying from 0.1 to 0.9) were successfully grown by the Bridgman method. D.C. Hall effect and temperature dependence of π of these crystals were measured. π at 300°K is found to decrease and the carrier concentration to increase approximately exponentially with x. The temperature dependence of π shows that the conduction is thermally activated. At low temperature (< 300 °K) extrinsic conduction is dominant with the value of the activation energy decreasing from 0.065 to 0.006 eV as x increases from 0.1 to 0.9. At higher temperature, intrinsic conduction is observed for alloys with x = 0.4, 0.5, 0.6 and 0.8 yielding the values of intrinsic band gaps (E_g). The values of E_g are found to decrease with x.     

Optical, electrical and thermal studies on (As2Se3)3−x(As2Te3)x glasses

Optical properties and conductivity of glassy (As₂Se₃)_3−x(As₂Te₃)_x were studied for 0 ≤ x ≤ 3. The films of the above mentioned compound were prepared by thermal evaporation with thickness of about 250 nm. The optical-absorption edge is described and calculated using the non-direct transition model and the optical band gap is found to be in the range of 0.92 to 1.84 eV. While, the width of the band gap tail exhibits opposite behaviour and is found to be in the range of 0.157 to 0.061 eV, this behaviour is believed to be associated with cohesive energy and average coordination number. The conductivity measurement on the thin films is reported in the temperature range from 280 to 190 K. The conduction that occurs in this low-temperature range is due to variable range hopping in the band tails of localized states, which is in reasonable agreement with Mott's condition of variable range hopping conduction. Some parameters such as coordination number, molar volume and theoretical glass transition temperature were calculated and discussed in the light of the topological bonding structure.     

Photoelectrochemical investigation on spray depositedn-CdIn2S4 thin films

Polycrystalline thin films ofn-CdIn₂S₄ have been spray deposited onto amorphous and fluorinedoped tin oxide (FTO) coated glass substrates at the optimized substrate temperature of 380°C. The films were characterized by X-ray diffraction (XRD) and optical absorption studies. XRD studies revealed that the films were polycrystalline with spinel cubic structure. The optical absorption studies showed the band gap energy to be 2·14 eV. Photoelectrochemical (PEC) investigations were carried out using cell configurationn-CdIn₂S₄/1 M NaOH+1 M Na₂S+1 M S/C. Using Butler model, the optical band gap and minority carrier diffusion length (L _P) were found to be 2·22 eV and 0·07 μm, respectively. Gartner’s model was used to calculate the minority carrier diffusion length and the donor concentration (N _D) for CdIn₂S₄ films at three different wavelengths.N _D was found to be of the order of 10¹⁶ cm⁻³.     

Effect of annealing temperature on structural,optical and humidity sensing properties of indium tin oxide (ITO) thin films

Tin doped indium oxide (ITO) thin films were prepared by sol–gel spin coating method with In (NO₃)·3H₂O and SnCl₄·5H₂O as indium and tin sources, respectively. The as deposited samples were annealed at various temperature such as, 300, 400, 500 and 600?°C for 2 h in ambient atmosphere. The grown ITO thin films are polycrystalline in nature with cubic structure of In₂O₃ with the space group La3 and the results are in good agreement with the standard JCPDS data (card no#06-0416). In addition crystalline size increases with increasing annealing temperature from 25 to 55 nm. Polycrystalline with uniform smooth surface was observed by SEM micrographs. The optical band gap energy was found to be decreased from 3.85 to 3.23 eV as the annealing temperature is increased from 300 to 600?°C. The humidity sensing performance (high sensitivity and fast response time) was significantly improved for 600?°C thin films samples, which is probably due to smaller energy band gap and physisorption between the water molecules and the surface of the thin films. The films were further characterized by PL and EDS analysis. The effect of temperature on humidity sensing mechanism of ITO thin films is also discussed.     

Space charge limited conduction and determination of density of localised states in semiconductor cobalt doped TiO2

Abstract

The space charge limited conduction (SCLC) mechanism in Co doped TiO₂ has been investigated at different temperatures. At lower electric fields, ohmic behaviour is observed while at higher electric fields nonohmic behaviour is observed. The results obtained confirm the presence of SCLC in Co doped TiO₂. The electronic parameters such as the position of the Fermi level above the valence band edge E _F, the density of states in valence band N _V and effective mass of holes m _h were found as 12·32 meV, 1·26 × 10¹⁵ m^?3 and 1·33 × 10^?7 m_e, respectively. The distribution of localised states in the forbidden band gap of the Co doped TiO₂ was characterised by current–voltage measurements and the density of localised states near the Fermi level N(E_F) was found to be 2·11 × 10¹⁷ eV^?1 m^?3.     

Conduction à l'obscurité dans les couches minces de CdTe,type N

In order to determine the trap levels in the band gap of n-type CdTe thin films, we have studied the dark conduction in high electric fields. Our study enables us to propose values for the density of a uniform distribution of traps located in the band gap, η_t ≈ 5 × 10¹³ eV^-1, and for the position of the Fermi level, E_c - E_F0 ≈ 0,666 eV. We have also determined the width and the position of this distribution in the band gap to be ΔE = E₂ - E₁ ≈ 0,107 eV and E_c - E₁ ≈ 0,5 eV respectively. These values enable us to construct a possible model for the band gap.     

Grain size dependent optical band gap of CdI2 films

The thermally evaporated stoichiometric CdI₂ films show goodc-axis alignment normal to substrate plane for film thickness up to 200 nm. The optical absorption data indicate an allowed direct interband transition across a gap of 3.6 eV in confirmation with earlier band structure calculations. However, part of the absorption data near band edge can be fitted to an indirect band gap of 3 eV. The dependence of band gap on film thickness (> 200 nm) can be explained qualitatively in terms of decreasing grain boundary barrier height with grain size.     

Electrical conductivity of single crystal nickel tungstate

Measurements of a.c. and d.c. electrical conductivity in crystals of nickel tungstate, in the temperature range 300 to 1100 K, are presented. NiWO₄ is found to be a semiconductor with a band gap of 2.10 eV. The nature of the electrical conduction is discussed in the light of various conduction models.