Thermal Band Gap Research Articles

Growth and humidity sensing properties of plate-like CuO nanostructures

This paper describes a simple, low-temperature and cost-effective liquid-phase method to synthesize homogenous plate-like CuO nanostructures and their sensitivity to humidity. Scanning electron microscopy illustrated that the synthesized CuO nanoplates have thicknesses of ~100 nm. X-ray diffraction measurements showed that the CuO nanostructures have high crystallinity with monoclinic crystal structure preferentially in and directions. From the temperature-dependant dark electrical resistivity measurements, the ionization energies of the impurity levels and thermal band gap energies of the films are found as 0.30, 0.32 and 1.37, 1.39 for as-synthesized and annealed films, respectively. Gas sensing characteristics of the films were investigated for different concentrations of humidity, isopropyl alcohol, methanol, ethanol, chloroform and acetone vapours. It was found that the sensor is sensitive to humidity but not to the other volatile organic compounds.

Structural, Thermal and Magnetic Analysis of Co<sub>2</sub>FeO<sub>4 </sub>Spinel Oxide Synthesized by Co-Precipitation Process

Co2FeO4spinel oxide nano powder was synthesized by co-precipitation process. Samples were annealed at different temperatures of 500, 800 and 900oC and the phase purity were also analyzed. As-prepared sample has two spinel phases and on annealing at 900οC single phase compound formed. The crystallite size of as-prepared is found to be 11.9 nm. The crystallite size increased with increase in annealing temperature to a maximum of 17.4 nm for 900 °C. Samples were subjected to necessary characterization for finding thermal, magnetic and optical band gap.

Low band gap polymers for application in solar cells: synthesis and characterization of thienothiophene–thiophene copolymers

In this paper we present the synthesis and characterization of two novel copolymers obtained from a bithiophene unit carrying octylsulfanyl side chains and thienothiophene units substituted with keto (PK) or ester (PE) groups. Their structural, electrochemical and photophysical properties were investigated by gel permeation chromatography (GPC), thermogravimetric analysis (TGA), NMR, UV-visible-NIR spectroscopy, cyclic voltammetry (CV) and atomic force microscopy (AFM). They possess good solubility in common organic solvents, filmability, proneness to form π-stacks, moderate solvatochromism, good thermal stability and low band gap energy. They were tested as donor materials in combination with [70]PCBM (electron acceptor) in bulk-heterojunction polymer solar cells. The geometry of the devices is: glass/ITO/PEDOT:PSS/copolymer:[70]PCBM/Ca/Al. The external quantum efficiency curve of the best device, realized using a blend of PK : [70]PCBM, 1 : 2 weight ratio, shows a broad response from 350 to 1000 nm. The power conversion efficiency under 100 mW cm−2 AM 1.5G illumination is greater than 1%.

Naphtho[1,2-b:5,6-b′]dithiophene-Based Donor–Acceptor Copolymer Semiconductors for High-Mobility Field-Effect Transistors and Efficient Polymer Solar Cells

Two donor–acceptor (D–A) copolymers, PzNDTDTBT and PzNDTDTBO, using 4,9-bis(2-ethylhexyloxy)naphtho[1,2-b:5,6-b′]dithiophene as an electron-rich unit and benzodiathiazole (BT) or benzoxadiazole(BO) as an electron-deficient one, were designed and synthesized via a Pd-catalyzed Stille-coupling method. The acceptor units were varied from BT to BO for adjusting the energy levels and optimizing the structures of polymers. Both copolymers possess good solubility, high thermal stability, broad absorption, and low bandgap and exhibit not only high field-effect mobilities but also high photovoltaic properties. The hole mobilities reach 0.43 and 0.34 cm2 V–1 s–1 for PzNDTDTBT and PzNDTDTBO film, respectively. Bulk heterojunction solar cells fabricated by using PzNDTDTBT or PzNDTDTBO as electron donor and PC71BM as acceptor show a power conversion efficiency of 3.22% and 5.07%, respectively, under illumination of AM1.5G, 100 mW cm–2. Both the hole mobilities and PCEs are among the highest values in the current reports based on naphthodithiophene-based polymers, indicating that “zigzag” naphthodithiophene-based D–A copolymers are very promising for application as solution-processable organic semiconductors in optoelectronic devices.

Sonochemically Synthesized 3D Assemblies of Close-Packed In2S3 Quantum Dots: Structure, Size Dependent Optical and Electrical Properties

Template-free conventional chemical and sonochemical approaches to 3D assemblies of indium(III) sulfide quantum dots were developed that allow deposition of strongly quantized cubic α-In2S3 nanocrystals close packed in thin film form. Our observation of metastable cubic structure at room temperature (instead of the thermodynamically most stable tetragonal β modification in the case of bulk material) was related to the very small crystal size. Because of heterogeneous sonochemical effects, the average crystal radius of the QD solids reduces from 2.5 to 2.0 nm upon sonification of the reaction system by continuous high-intensity ultrasound. Upon postdeposition annealing treatment, these values increase to 4.1 nm. Structural, optical and electrical properties of the synthesized QD solids were studied in details. The band gap energy value of 2.85 eV for the as-deposited QD solids in thin film form is strongly blue-shifted (by 0.85 eV) with respect to the value characteristic for a macrocrystalline specimen. In the case of as-deposited films by sonochemical approach, band gap value is 3.00 eV, indicating the possibility for further control of the optoelectronic properties of this material by sonochemical approach. Upon postdeposition thermal treatment at 150 and 200 °C, band gap energy red shifts to 2.20 and 2.00 eV were observed. Analysis of the size-quantization effects in the synthesized QD solids deposited in thin film form enabled us to estimate that the Bohr’s excitonic radius in the studied semiconductor lies in the range from 2.5 to 4.1 nm. The absence of clearly defined excitonic peaks in the absorption spectra of the studied QD assemblies was attributed to the size-distribution of the nanoparticles and to the interdot electronic coupling effects. Analysis of the charge carrier transport properties in the QD assemblies within the Kazmerski’s model indicated that the intercrystalline barrier height decreases by 0.04 eV upon thermal treatment of the films. Conductivity activation energy was found to be 0.82 eV, while the thermal band gap energy, calculated from the thermoelectrical measurements in the region where intrinsic conductivity mechanism is activated, was 2.22 eV. AFM measurements have shown that QD assemblies constituting the sonochemically deposited films show stronger tendency toward coagulation than those synthesized by conventional chemical approach.

Synthesis and characterization of copolymers based on benzotriazoles and different atom-bridged dithiophenes for efficient solar cells

A series of donor (D)–acceptor (A) alternating copolymers based on a benzotriazole (BTZ) acceptor and different dithiophene donors, such as dithieno[3,2-b:2′,3′-d]pyrrole (NT), dithieno[3,2-b:2′,3′-d]silole (ST) and cyclopenta[2,1-b:3,4-b′]dithiophene (CT), have been successfully synthesized to study the effect of bridging atoms on the properties for application in polymer solar cells (PSCs). The resulting copolymers exhibit good solubility in organic solvents, good thermal stability and low bandgaps. It is important to note that their absorption spectra, electronic energy levels as well as photovoltaic properties can be facilely modulated by changing the bridging atom of donor segments from N, Si to C. The UV-vis and cyclic voltammetry results show that PNTBTZ containing NT block has the best light-harvesting ability and the narrowest bandgap because of the strongest electron-donating property of NT unit compared to ST and CT. However, PSTBTZ exhibits the lowest lying HOMO energy level, which is beneficial to achieve the high Voc of the related polymer solar cells. The basic electronic structures of D–A model compounds of these copolymers were also studied by DFT calculations at the B3LYP/6-31G* level. The PSCs using these copolymers were fabricated and evaluated with a typical structure of ITO/PEDOT:PSS/copolymer:PC71BM/Ca/Al under the illumination of AM 1.5G, 100 mW cm−2. As expected, the PNTBTZ device has the highest Voc value among these PSCs for the deepest HOMO energy level. However, the device based on PCTBTZ with a C-bridged dithiophene (CT) donor segment exhibits the highest Jsc of 9.24 mA cm−2, FF of 0.64 and PCE of 4.02% due to the broadest and strongest spectral response as well as ideal morphology compared to PNTBTZ and PSTBTZ.

Effect of Oligothiophene π-Bridge Length on the Photovoltaic Properties of D–A Copolymers Based on Carbazole and Quinoxalinoporphyrin

A series of low-bandgap donor–acceptor (D–A) copolymers, P(C-T-QP), P(C-BT-QP), P(C-TT-QP), and P(C-TT-QP-Zn), using 2,7-carbazole (C) as an electron-rich unit and quinoxalino[2,3-b′]porphyrins (QP) or quinoxalino[2,3-b′]porphyrinatozinc(QP-Zn) as an electron-deficient unit with different length of oligothiophene π-bridges, were designed and synthesized via a Pd-catalyzed Stille-coupling method. The π-bridge between the C donor unit and the QP acceptor unit is thiophene (T) in P(C-T-QP), bithiophene (BT) in P(C-BT-QP), and terthiophene (TT) in P(C-TT-QP) or P(C-TT-QP-Zn). These copolymers possess good solubility, high thermal stability, broad absorption, and low bandgap ranging from 1.66 to 1.73 eV. The influence of the π-bridge and the central Zn ion on the electronic and photovoltaic properties was investigated and discussed in detail. It was found that the π-bridge played an important role in tuning the effective conjugation length and therefore significantly affected the molecular architecture and opt...

Semiconductor band gap localization via Gaussian function

To determine the band gap of bulk semiconductors with transmission spectroscopy alone is considered as an extremely difficult task because in the higher energy range, approaching and exceeding the band gap energy, the material is opaque yielding no useful data to be recorded. In this paper, by investigating the transmission of industrial GaSb wafers with a thickness of 500 µm, we demonstrate how these obstacles of transmission spectroscopy can be overcome. The key is the transmission spectrums’ derivative, which coincides with the Gaussian function. This understanding can be used to transfer Beers’ law in an integral form opening the pathway of band gap determinations based on mathematical parameters only. The work also emphasizes the correlation between the thermal band gap variation and Debye temperature.

Thermal stability and band gap study of heavy metal oxy-sulfide glass system

Novel oxy-sulfide glass system xPbS-(73 − x) Bi2O3–27B2O3 with 6.06 ≤ x ≤ 36.35 named lead sulfide bismuth borate (LSBB) was prepared using normal melt and quench-casting technique. Phase transition temperatures tg, tx, tpi, and tl were noted from the DTA curves. Glass transition temperature tg varied from 306 ± 2 to 336 ± 2 °C and the onset of crystallization temperatures tx was between 331 ± 2 and 402 ± 2 °C. The glasses melted in the range 523 ± 2 to 597 ± 2 °C. The ratio tg/tl showed that compositions reported conform to the two-third law of glass formation. Hruby’s coefficient Hr witnessed the thermal stability of the system and that LSBB4 was most stable and glass formability factor kgl showed that composition LSBB1–LSBB6 can easily corroborate vitreous state. The direct band gap energy varied from 1.56 to 3.07 eV, while indirect band gap energy for the fundamental absorption edge was 0.21–1.31 eV. Absorption edges obeyed Urbach rule. A broad range of band tailing was exhibited confirming amorphous state of the system.

Study of Electrical, Thermal and Optical Behavior of Polypyrrole Filled PVC:PMMA Thin Film Thermoelectrets

The present study reveals with study of electrical, thermal and optical band gap of polypyrrole filled PVC: PMMA thin films themoelctrets. Conducting polymer composites have attracted considerable interest in recent years because of their numeric applications in variety of electric and electronic devices. Polypyrrole has been regarded as one of the most studied conducting polymer because of its physical and electrical properties that have led to several applications such as solid state devices and electronics. The electrical conductivity of polypyrrole filled PVC: PMMA thermoelectrets have been studied. Electret effects in polymers can be produced by orientation of dipoles and /or trapping of charge carriers injected from electrodes as well as generated in the bulk of polymers. It has been shown by thermally stimulated discharge conductivity technique (TSDC) that the characteristics of electrets are very sensitive to the structure of electret forming materials. In this respect TSDC study of composite is likely to yield information about the extent of mixing between the components. Electrets prepared by composites have better charge storage capacity than the individual polymers. Measurements of TSD conductivity have been carried out at different polarizing fields. It has been observed that the conductivity of polypyrrole filled PVC: PMMA composite have been increased with increase in percentage of polypyrrole. The X-RD diffractogram reveals the amorphous nature of the films. The thermograms are plotted between logand temperature (10 3 /T).From the analysis of the absorption spectra the band gap of polypyrrole filled PVC: PMMA composite have been found to be lie in the range 1.8 eV to 3.3 eV. Thermal stability of polypyrrole filled PVC: PMMA was investigated by TGA/DSC. It is evident from the results that PPy filled polyblends are more stable.

Influence of the Triel Elements (M = Al, Ga, In) on the Transport Properties of Ca5M2Sb6 Zintl Compounds

The Zintl compound Ca_5Al_2Sb_6 has extremely low lattice thermal conductivity (<0.6 W/mK at 1000 K) and tunable electronic properties, making it a promising thermoelectric material for high temperature waste-heat recovery. The current study investigates trends in the chemical and transport properties of the Ca_5M_2Sb_6 compounds (M = Al, Ga, or In), revealing potential routes toward improved thermoelectric properties in this system. Here, we show that isoelectronic M-site substitutions can be used to “fine-tune” the electronic properties of the Ca_5M_2Sb_6 system, without inducing electronic doping effects. Electronic structure calculations reveal that the electronegativity of the M element is a good indicator for the energy level of M electronic states. The effects of M-site substitutions on the effective mass and band gap are reflected in measurements of the high temperature electronic properties of Ca_5M_2Sb_6 samples (M = Al, Ga, and In) which reveal increased hole mobility as well as a smaller thermal band gap in the Ga analogue, relative to Ca_5Al_2Sb_6 and Ca_5In_2Sb_6. Optical absorption measurements reveal a trend in the direct band gaps consistent with both calculations and transport measurements. Additionally, a direct benefit of substituting heavier elements on the Al site arises from the increased density and softer lattice, which leads to reduced sound velocity and lattice thermal conductivity.

Photoelectric characteristics of nanostructured hydrochemically deposited films of the Pb0.975Sn0.025Se substitutional solid solution

The photoelectric characteristics of Pb0.975Sn0.025Se solid solution films prepared by the hydrochemical codeposition of PbSe and SnSe with the subsequent heat treatment in air at 573–700 K have been investigated. The thermal and optical band gaps, the temperature coefficient of the optical band gap, the dark resistance, the volt-watt sensitivity, and the range of spectral sensitivity have been determined in the temperature range of 220–300 K. It has been found that, after heat treatment below 573 K, the films of the Pb0.975Sn0.025Se solid solutions possess metallic conductivity, while being heat treated at elevated temperatures, they become semiconductors with p-type conductivity. The composition of the solid solution is independent of the heat treatment temperature; it is formed during deposition.

Hydrochemical synthesis, structure, semiconductor properties of films of substitutional Pb1 − xSnxSe solid solutions

The region of lead and tin selenide co-deposition using selenourea has been examined by calculation of the ionic equilibrium in the citrate–ammonia system. The formal kinetic equations for the rates of formation of the lead and tin selenides have been determined. Thin films of substitutional Pb1−xSnxSe (0<x<0.093) solid solutions were produced from aqueous solutions. The composition, crystal lattice spacing, thermal band gap and conductivity of the films have been analysed.

Changes in thermal conductivity and bandgap of SiC single crystals in accordance with thermal stress

Thermal stress according to growth temperature and surface region of single crystal SiC was analyzed using ABAQUS simulation. Three 6H SiC single crystal wafers grown with the PVT method under the same growth conditions by different companies were used to analyze the changes in properties caused by thermal stress. All three wafers differed in diffraction distance. The wafer with the higher tensile stress from thermal stress showed a larger diffraction distance. Also the impact of thermal stress on crystallinity and defect density was analyzed. Our results showed that the wafer with the higher thermal stress had lower crystallinity and more defects, and we also confirmed that the wafer had a lower thermal conductivity and bandgap.

Anisotropy of plasma reflection of solid solutions (Bi2 − x Sb x )Te3 (0 &lt; x &lt; 1) in the Temperature Range 78–293 K

We study IR polarized reflection spectra of solid solutions (Bi2−xSbx)Te3 (0 < x < 1) in the temperature range 78–293 K and in the excitation range of plasma oscillations of free charge carriers. We reveal that, as the content of Sb2Te3 in the solid solution increases from 0 to 50%, the anisotropy of both plasma frequencies and magnetic susceptibility vary insignificantly. This indicates that, at 293 K, these crystals do not show significant changes in the composition of groups of free carriers near the chemical potential level. We have found that to describe the observed anisotropy of plasma frequencies, it is necessary to take into account the effect of heavy holes of the valence band. This is also confirmed by the results of the study of the temperature behavior of plasma reflection spectra, which exhibit regularities that are similar to anomalous temperature changes of the Hall coefficient. We show that a decrease in plasma frequencies with increasing temperature cannot be described by an increase in the effective mass of charge carriers in accordance with the expression m* ∼ T0.17, which was obtained upon interpretation of temperature dependences of a number of kinetic coefficients. We use the empirical Moss rule ɛ∞2Eg to estimate the rate of decrease of the thermal band gap dEg/dT = −1.6 × 10−4 eV/deg in a crystal that contains 25% Sb2Te3.

Low-temperature investigations of chemically deposited films of substitutional solid solutions based on lead and tin (II) selenides

Photoelectric characteristics of films of the Pb0.902Sn0.098Se solid solution based on the four-layer (SnSe-PbSe)2 composition obtained by layer-by-layer hydrochemical synthesis with subsequent heat treatment in air within the temperature range 523–700 K are investigated. The measurements are performed in the temperature range 220–300 K. The thermal and optical band gaps, temperature coefficient of the optical band gap, dark resistance, volt-watt sensitivity, and spectral characteristics are determined. It is established that, at heat treatment temperatures below 573 K, the conductivity of the four-layer (SnSe-PbSe)2 compositions is characteristic of metals, while at higher temperatures, it is characteristic of p-type semiconductors.

Analysis of thermal band gap variations of PbS quantum dots by Fourier transform transmission and emission spectroscopy

Fourier transmission and emission spectroscopy was employed in the range from 5 to 300 K to measure the thermal band gap shift of 4.7 nm PbS quantum dots. The analytical comparison of fits carried out with the expressions of Varshni and Fan revealed limited accuracy of the Varshni fitting parameters. Evidence is presented that transmission spectroscopy in conjunction with the Fan model concurs with the microscopic material features, resulting in a tool to determine intrinsic properties of quantum dots.

Mechanochemical synthesis and characterization of xIn2O3·(1 − x)α-Fe2O3 nanostructure system

Indium oxide-doped hematite xIn2O3·(1 − x)α-Fe2O3 (x = 0.1–0.7) nanostructure system was synthesized using mechanochemical activation by ball milling and characterized by XRD, simultaneous DSC–TGA, and UV/Vis/NIR. The microstructure and thermal behavior of as obtained system were dependent on the starting In2O3 molar concentration x and ball milling time. XRD patterns yielded the dependence of lattice parameters and grain size as a function of ball milling time. After 12 h of ball milling, the completion of In3+ substitution of Fe3+ in hematite lattice occurs for x = 0.1, indicating that the solid solubility of In2O3 in hematite lattice is extended. For x = 0.3, 0.5, and 0.7, the substitutions between In3+ and Fe3+ into hematite and In2O3 lattice occur simultaneously. The lattice parameters a and c of hematite and lattice parameter a of indium oxide vary as a function of ball milling time. The changes of these parameters are due to ion substitutions between In3+ and Fe3+ and the decrease in the grain sizes. Ball milling has a strong effect on the thermal behavior and band gap energy of the as-obtained system. The hematite decomposition is enhanced due to the smaller hematite grain size. The crystallization of hematite and In2O3 was suppressed, with drops of enthalpy values due to the stronger solid–solid interactions after ball milling, which caused gradual In3+–Fe3+ substitution in hematite/In2O3 lattices. The band gap for hematite shifts to higher energy value, while that of indium oxide shifts to lower energy value after ball milling.

Growth of homogenous CuO nano-structured thin films by a simple solution method

This paper describes a simple, low temperature and cost effective solution method to synthesize homogenous cupric oxide (CuO) nano-structured thin films. By this method dense and continuous CuO films with good crystallinity can be prepared in a short time, e.g., in 15 min. The scanning electron microscopy illustrated that the synthesized CuO plate-like nanostructures have RMS thicknesses of 100 nm. The XRD measurements showed that the synthesized CuO nanostructures have a high crystallinity with monoclinic crystal structure preferentially in ( 1 ¯ 1 1 ) and (1 1 1) directions. From the temperature dependant dark electrical resistivity measurements the ionization energies of the impurity levels and thermal band gap energies of the films were calculated.

Structure and properties of low-phonon antimony glasses and nano glass-ceramics in K 2O–B 2O 3–Sb 2O 3 system

A series of new low-phonon and low softening point monolithic antimony glasses and nano glass-ceramics (crystallite diameter 7–16 nm) containing as high as 90 mol% Sb 2O 3 were prepared in the ternary K 2O–B 2O 3–Sb 2O 3 (KBS) system. Infrared spectra and X-ray analysis establish that their structures closely resemble valentinite form of Sb 2O 3. The phonon energy has been found to be 592–602 cm −1. The softening point ( T s), glass transition temperature ( T g), coefficient of thermal expansion (CTE), dielectric constant ( ε) and optical band gap ( E opt) have been found to vary in the ranges 331–392 °C, 234–264 °C, 201–222 × 10 −7 K −1, 12.4–14.5 and 3.15–3.22 eV, respectively. These properties are found to be controlled by the covalent character and optical basicity of prepared samples. The average electronic polarizability of the oxide ions and optical basicity are found to increase while interaction parameter and metallization criterion decrease with increase in Sb 2O 3 content.