Electronic properties of Al/DNA/p-Si MIS diode: Application as temperature sensor

The current-voltage (I-V) measurements were performed in the temperature range (200-300 K) on Al/DNA/p-Si Schottky barrier type diodes. The Schottky diode shows non-ideal I-V behaviour with ideality factors n equal to 1.34 ± 0.02 and 1.70 ± 0.02 at 300 K and 200 K, respectively, and is thought to have a metal-interface layer-semiconductor (MIS) configuration. The zero-bias barrier height Φ_b determined from the I-V measurements was 0.75 ± 0.01 eV at 300 K and decreases to 0.61 ± 0.01 eV at 200 K. The forward voltage-temperature (V_F-T) characteristics were obtained from the I-V measurements in the temperature range 200-300 K at different activation currents (I_F) in the range 20 nA-6 μA. The V_F-T characteristics were linear for three activation currents in the diode. From the V_F-T characteristics at 20 nA, 100 nA and 6 μA, the values of the temperature coefficients of the forward bias voltage (dV_F/dT) for the diode were determined as −2.30 mV K⁻¹, −2.60 mV K⁻¹ and −3.26 mV K⁻¹ with a standard error of 0.05 mV K⁻¹, respectively.     

Polyaniline/TiO2 solar cells

We have used sol–gel spin-coating method to prepare nano-crystalline TiO₂ films for two kinds of polyaniline-based solar cell. The junctions thus obtained show rectifying behaviour. Their I–V characteristics indicate that a built-in electric field has been created at the nano-crystalline TiO₂/polyaniline interface. Their I–V characteristics and energy conversion efficiency under simulated solar irradiation are measured and discussed.     

To the theory of Ostwald ripening in metallic alloys

The mechanism of growth (dissolution) of nanoparticles controlled simultaneously by the matrix diffusion, diffusion along dislocations, and the rate of the passage of atoms through the interphase boundary (by the rate of formation of interatomic bonds) has been studied in alloys of CuNiAl and AlLi metallic systems at the stage of Ostwald ripening. An expression for the nanoparticle-size distribution function has been obtained, which was calculated based on the dependence of the rate of growth on three fluxes, i.e., diffusion flux j_V, dislocation flux j_d, and kinetic flux j_i. A comparison of the experimental histograms with the theoretically calculated curves indicates their good agreement, which can indicate the opportunity of the realization in practice of the proposed mechanism of growth of nanoparticles in alloys of metallic systems.     

On the mechanism of current-transport in Cu/CdS/SnO2/In-Ga structures

The structural and optical properties of CdS films deposited by evaporation were investigated. X-ray diffraction study showed that CdS films were polycrystalline in nature with zinc-blende structure and a strong (1 1 1) texture. The study has been made on the behavior of Cu/n-CdS thin film junction on SnO₂ coated glass substrate grown using thermal evaporation method. The forward bias current-voltage (I-V) characteristics of Cu/CdS/SnO₂/In-Ga structures have been investigated in the temperature range of 130-325 K. The semi-logarithmic lnI-V characteristics based on the Thermionic emission (TE) mechanism showed a decrease in the ideality factor (n) and an increase in the zero-bias barrier height (Φ_Bo) with the increasing temperature. The values of n and Φ_Bo change from 8.98 and 0.29 eV (at 130 K) to 3.42 and 0.72 eV (at 325 K), respectively. The conventional Richardson plot of the ln(I_o/T²) vs q/kT shows nonlinear behavior. The forward bias current I is found to be proportional to I_o(T)exp(AV), where A is the slope of ln(I)-V plot and almost independent of the applied bias voltage and temperature, and I_o(T) is relatively a weak function of temperature. These results indicate that the mechanism of charge transport in the SnO₂/CdS/Cu structure in the whole temperature range is performed by tunneling among interface states/traps or dislocations intersecting the space-charge region. In addition, voltage dependent values of resistance (R_i) were obtained from forward and reverse bias I-V characteristics by using Ohm's law for each temperature level.     

Temperature dependent current-voltage and capacitance-voltage characteristics of chromium Schottky contacts formed by electrodeposition technique on n-type Si

The variations in the electrical properties of Cr Schottky contacts formed by electrodeposition technique on n-type Si substrate have been investigated as a function of temperature using current-voltage (I-V) and capacitance-voltage (C-V) measurements in the temperature range of 80-320 K by steps of 20 K. The basic diode parameters such as ideality factor (n) and barrier height (Φ_b) were consequently extracted from the electrical measurements. It has been seen that the ideality factor increased and the barrier height decreased with decreasing temperature, when the I-V characteristics were analyzed on the basis of the thermionic emission (TE) theory. The abnormal temperature dependence of the Φ_b and n and is explained by invoking two sets of Gaussian distribution of barrier heights at 320-200 K, and 180-80 K. The double Gaussian distribution analysis of the temperature-dependent I-V characteristics of the Cr/n-type Si Schottky contacts gave the mean barrier heights of 0.910 and 0.693 eV and standard deviations (σ_s) of 109 mV and 72 mV, respectively. Then, these values of the mean barrier height have been confirmed with the modified ln(I₀/T²) − q²/2k²T² versus 1/T plot which belongs the two temperature regions.     

Photo-induced magnetization in copper(II) octacyanomolybdate(IV)

The photomagnetic effect of the mixed-valence compound Cu₂^II[Mo^IV(CN)₈]·7.6H₂O (Cu^II; 3d⁹, S=1/2, Mo^IV; 4d², S=0) (1) was investigated. The UV–VIS spectra showed the intervalence transfer (IT) band between Mo^IV–CN–Cu^II and Mo^V–CN–Cu^I around 500 nm. When the paramagnetic compound 1 was irradiated by a filtered blue light (400–430 nm, 3 mW/cm²) of a Xe lamp at 5 K, the irradiated sample exhibited a spontaneous magnetization with a Curie temperature of 17 K. By annealing the irradiated sample above 200 K, the magnetic property of this sample returned to the initial state. This photo-induced magnetization is explained by the following mechanism. The excitation of the IT band causes the electron transfer from Mo^IV (S=0) to Cu^II (S=1/2), producing a mixed-valence isomer of Mo^V (S=1/2)–CN–Cu^I (S=0). However, half of the copper ions remain as Cu^II ions due to the stoichiometric limitation and hence the irradiated 1 is expressed as Cu^ICu^II[Mo^V(CN)₈]·7.6H₂O. The unpaired electrons on Mo^V (S=1/2) ions and those on Cu^II (S=1/2) ions in the irradiated compound interact ferromagnetically, giving rise to the spontaneous magnetization.     

Humidity-dependent characteristics of thin film poly(3,4-ethylenedioxythiophene) field-effect transistor

π-Conjugated poly(3,4-ethylenedioxythiophene) (PEDOT) based field-effect transistors (FETs) were fabricated in this study. A thin PEDOT layer (thickness ≈500 nm) with the desired pattern was formed as an active and a gate electrode by a vapor polymerization of 3,4-ethylenedioxythiophene (EDOT) on the photolithographically patterned ferric p-toluenesulfonate (FTS) oxidant layer. Crosslinked poly(vinyl cinnamate) (PVCN) insulating layer was formed by spin-coating and UV crosslinking. The currents (I_ds) of the PEDOT active channel decreased with increasing gate bias (V_g), implying p-type FET. The dc conductivities (σ_dc) and I_ds of the PEDOT active channel were measured as a function of V_g under various relative humidities (RHs) ranging from 0% to 55%. The σ_dc and the I_ds of the PEDOT channel rapidly decreased with increasing positive V_g under humid conditions, while those of the PEDOT channel showed no change with V_g in vacuum, i.e. 0% RH. The threshold gate bias, which is defined as the onset of the curve of I_ds versus V_g, rapidly decreased with increase of RH. The moisture absorbed in the PEDOT active channel may assist the dedoping of the system by the screening and/or the relative separation of constituent ions from PEDOT chain upon applying a positive V_g. When the positive V_g was removed (V_g = 0), the σ_dc of the channel recovered slowly and a relatively long relaxation time was observed, which implies that the PEDOT active layer was slowly redoped. For the PEDOT based FET, we observed the moisture assisted dedoping and the redoping processes of the PEDOT channel upon applying and removal of positive V_g under the humid conditions.     

Electric and dielectric behavior of CaCu3Ti4O12-based thin films obtained by soft chemical method

CaCu₃Ti₄O₁₂ (stoichiometric) and Ca_1.1Cu_2.9Ti₄O₁₂ (non-stoichiometric) thin films have been prepared by the soft chemical method on Pt/Ti/SiO₂/Si substrates, and their electrical and dielectric properties have been compared as a function of the annealing temperature. The crystalline structure and the surface morphology of the films were markedly affected by the annealing temperature and excess calcium. The films show frequency-independent dielectric properties at room temperature which is similar to those properties obtained in single-crystal or epitaxial thin films. The room temperature dielectric constant of the 570-nm-thick CCTO thin films annealed at 600 °C at 10 kHz was found to be 124. The best non-ohmic behavior (α = 12.6) presented by the film with excess calcium annealed at 500 °C. Resistive hysteresis on the I-V curves was observed which indicates these films can be used in resistance random access memory (ReRAM).     

I–V characteristics of dark and illuminated PPV-PCBM blends solar cells

The effect of temperature and composition on the I–V characteristics of blends of (poly(2-methoxy-5-(2′-ethyl-hexyloxy)-l,4-phenylene vinylene) (MEH-PPV), and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) sandwiched between a transparent indium tin oxide (ITO) electrode and an Al backside contact (solar cell structure) are investigated. The observed dark I–V curves agree well with the trap-controlled space charge limited transport theory. The illuminated I–V curves do not obey the theory of space charge limited currents. The short-circuit currents in the illuminated samples depend strongly on the composition. The current is maximum for the 80% PCBM concentration in the blend. The open circuit voltage also depends on the composition. It is maximum for 0% PCBM concentration and decreases monotonically with PCBM concentration. This might lead to the conclusion that the open circuit voltage is due to internal bulk heterojunctions (BHs). However, we have pointed out that this conclusion, arrived at by earlier workers also, is not necessarily valid. The superposition principle is not valid but the discrepancy is small.     

Devitrification behavior and some electrical properties of GeSeTl chalcogenide glass

The activation energy of crystallization E_c of GeSe₃Tl_0.3 and GeSe₄Tl_0.3 chalcogenide glasses was calculated from the shift of the DTA exothermic peaks with changing the heating rate. Values of E_c were calculated by different methods and it was found that it decreases with increasing Se content.Amorphous thin films of the present glasses were prepared using thermal evaporation technique. Electrical conductivity and I-V characteristics have been studied at temperatures below the glass transition temperature T_g and over a wide range of thickness (57.4-680 nm). The dc conductivity was linear in the Arrhenius Plot in the considered temperature range and increases with the increase of Se content. The activation energy for the dc conduction ΔE_σ is decreased with increasing Se content and film thickness. The observed compositional dependence of ΔE_σ has been correlated with the increase of weak Se-Se bond density at the expense of strong Ge-Se bond density. I-V characteristic curves indicates the memory type switching phenomenon for these compositions and it is explained in accordance with the electrothermal model initiated from Joule heating of a current channel.     

Why superconducting transition of polycrystalline β-ET2I3 in surface conducting films is observed below 8 K?

An onset of the superconducting transition in surface conducting films of polycarbonate with β-ET₂I₃ (ET: bis (ethylenodithio)tetrathiafulvalene) is observed at temperature(5–6 K) lower than that for single crystals (8 K). It is shown using X-ray diffraction technique that the interlayer spacing d in β-ET₂I₃ microcrystals in polymer film decreases stronger with temperature than in crystals without polymer. This is considered as an important factor suppressing the transition to superconductivity. It is assumed that stronger dependence d(T) in β-ET₂I₃ in polymer films is related to the stress induced by the shrinkage of the polymer matrix.     

Crystal growth and X-ray structure determination of Rb3Bi2I9

The compound Rb₃Bi₂I₉ was synthesized by melting a stoichiometric mixture of RbI and BiI₃ and annealing for 1 month some degrees below the melting temperature of 785±5K; a large single crystal of the title compound has been grown from the melt by directional solidification using the vertical Bridgman-Stockbarger technique. X-ray structure determination of the compound involved single-crystal diffractometry, direct and Patterson methods and least-squares refinement. Rb₃Bi₂I₉ exhibits a new type of crystal structure, crystallizing in the monoclinic space group Pc with unit-cell parameters a=14.690(4), b=8.195(2), c=25.645(6) Å, β=125.39(2)°, V=2517(2) ų, Z=4. The unit-cell of Rb₃Bi₂I₉ has six pseudohexagonal RbI₃ layers which are stacked along the direction near to the normal of the XY plane. Each BiI₆ octahedron shares three corners with three other octahedra to form parallel Bi₂I₉ corrugated layers intersecting Z axis of the structure at an angle near 120°.     

The dependence of lamellar spacing on growth rate and temperature gradient in the lead–tin eutectic alloy

Pb–Sn eutectic samples were directionally solidified under an argon atmosphere in a Bridgman type furnace in order to determine the dependence of lamellar spacing, λ_E, on the growth rate, V, the temperature gradient, G and the cooling rate, GV. Six samples were solidified with a constant V but different G and five samples were solidified with constant G but different V. The samples were solidified up to 10–12 cm length to ensure that the steady-state condition was obtained, and then quenched. Grinding, polishing and etching techniques were applied to longitudinal and the transverse sections of the samples to observe λ_E using an olympus BH-2 optical and a JEOL JSM 5400 scanning electron microscopy. Approximately 300–500 transverse lamellar spacings and 60–100 longitudinal lamellar spacings were measured and least-squares analysis was used to obtain the relationships between λ_E and V, G and GV. It was found that the value of λ_E decreases as the values of V, G and GV increase and that the results are in good agreement with the results of the previous work and with the Jackson–Hunt eutectic theory.     

Charge transport mechanism and the effect of poling on the current-voltage characteristics of conducting polyaniline–BaTiO3 composites

Conducting polyaniline–barium titinate (PANI–BaTiO₃) composites have been synthesized by `in situ' deposition technique by placing the fine grade powder of BaTiO₃ (prepared by conventional sol–gel technique) in the polymerization reaction mixture of aniline — finally yielding the Polyaniline–BaTiO₃ composites. The charge transport process in these composites have been studied by sandwiching the pellets of these composites between the silver and platinum electrodes. It presumably suggests presence of a nonlinear space charge controlled charge transport process. Also, a large hysteresis in the I–V characteristics of these composites has been observed which also supports the space charge controlled conduction. The samples were poled by applying a constant voltage for a fixed time and the I–V characteristics were traced before and after poling. Some striking differences have been observed in the I–V characteristics before and after poling. These have been explained on the basis of creation of an internal electric field due to poling which in turn opposes the externally applied electric field.     

The calculation of electronic parameters of an Ag/chitin/n-Si Schottky barrier diode

We have formed polymeric organic compound chitin film on n-Si substrate by adding a solution of polymeric compound chitin in N,N-dimethylacetamide and lithium chloride on top of an n-Si substrate and then evaporating solvent. It has been seen that the chitin/n-Si contact has demonstrated clearly rectifying behavior and the reverse curves exhibit a weak bias voltage dependence by the current–voltage (I–V) curves studied at room temperature. The barrier height and ideality factor values of 0.959 eV and 1.553, respectively, for this structure have been obtained from the forward bias I–V characteristics. Furthermore, the energy distribution of the interface state density located in the semiconductor band gap at the chitin/n-Si substrate in the energy range from (E_c − 0.897) to (E_c − 0.574) eV have been determined from the I–V characteristics. The interface state density, N_ss, ranges from 5.965 × 10¹² cm⁻² eV⁻¹ in (E_c − 0.897) eV to 1.706 × 10¹³ cm⁻² eV⁻¹ in (E_c − 0.574) eV and has an exponential rise with bias this energy range.     

Influence of Deformation on the Energy Spectrum and the Optical Properties of Fullerene C<Subscript>20</Subscript> within the Hubbard Model

The anticommutative Green’s functions and energy spectra of fullerene C₂₀ with symmetry groups I_h, D_5d, and D_3d have been obtained in analytical form within the Hubbard model in the mean-field approximation. The methods of group theory have been used to classify energy states and identify allowed transitions in the energy spectra of C₂₀.     

Carrier transport properties of aluminum oxide/polypyrrole films doped with naphthalene-1,5-disulfonic acid

The electrical structure of the Al/Al₂O₃/PPy-NDSA/Au has been investigated by means of current–voltage (I–V) and capacitance–voltage (C–V) measurements, in a temperature range of 90–350 K. The forward C–V measurements have been carried out in the range of frequency of 1 kHz to 20 MHz. The effects of series resistance, interfacial layer and interface states on I–V and C–V characteristics are investigated. At high current densities in the forward direction, the series resistance effect has been observed for voltages greater than 0.7 V. The analysis of I–V characteristics based on the thermionic emission mechanism has been explained by the assumption of a Gaussian distribution of barrier heights, due to barrier height inhomogeneities that prevail at the interface. It has been observed that the forward C–V plot exhibits a peak, whose position shifts towards lower voltages and that decreases with increasing frequency. The non-linearity of 1/C² versus V plot at high frequency was explained with the assumption that only some of the interface states follow the applied ac signal. Surface analysis by X-ray photoelectron spectroscopy (XPS) was performed in order to evaluate the chemical states of the constituents of the Al/Al₂O₃/PPy-NDSA/Au sample.     

Electrical and photovoltaic properties of FeTPPCl/p-Si heterojunction

Hybrid heterojunction cell based on thermally evaporated 5,10,15,20-tetraphenyl-21H, 23H-porphine iron (III) chloride (FeTPPCl) as the organic semiconductor and p-Si wafer as the inorganic semiconductor have been investigated. This device showed rectification behaviour like diode. The conduction mechanisms and the diode parameters have been studied using current–voltage (I–V) characteristics in the temperature range (298–373 K) and capacitance–voltage (C–V) characteristics at room temperature. This cell exhibited photovoltaic characteristics with a short-circuit current (I_sc) of 2.8 mA, an open-circuit voltage (V_oc) of 0.475 V, a fill factor FF = 32%.     

Magnetostriction effect of Co substitution in the Nd6Fe13Si intermetallic compound

We studied the magnetostriction of Nd₆Fe_13−xCo_xSi (x = 0, 1) intermetallic compounds with tetragonal Nd₆Fe₁₃Si-type structure, using the strain gauge method in the temperature range of 77–600 K under applied magnetic fields up to 1.5 T. The anisotropic magnetostriction (Δλ) versus temperature of the studied samples has shown almost similar field-dependence behavior. Below the spin reorientation temperature (T_SR), Δλ changes its sign from positive to negative value at an applied threshold field which increases with decreasing temperature. This behavior may originate from the reduction of the magnetocrystalline anisotropy with temperature. It is also observed that absolute value of Δλ increases by Co substitution. On the other hand, the volume magnetostriction (ΔV/V) versus field shows different behavior. The ΔV/V curves of Nd₆Fe₁₂CoSi tend to have a nearly quadratic dependence on applied field near magnetic ordering temperature as expected for the parastrictive behavior. The temperature dependence of magnetostriction values is discussed based on the magnetostriction relation of the tetragonal structure to determine the signs of some of magnetostriction constants for these polycrystalline compounds.