Heterojunction Theory Research Articles

Investigation of Ag nanoparticles decorated Ni-Al layered double hydroxide as a gas sensor

Chemiresistive gas sensors based on LDH structure were investigated for the detection of widely used volatile organic compounds (VOCs) including methanol, ethanol, and acetone at room temperature. In order to increase the surface active sites and subsequently improve the detection sensitivity of gas molecules, the use of Ag decoration method along with the creation of porosity on the surface of LDH structures were investigated. In this method, methanol solvent was used as a hole scavenger and UV radiation as a catalyst for binding Ag to the surface of LDH sheets. The microstructure and morphology of the composites were investigated by FESEM, HRTEM, EDS, EDS mapping, XRD, and MS analysis. The investigated parameters included relative humidity, calibration curve, repeatability, selectivity, and response/recovery times. According to the obtained results, the creations of surface corrosion by methanol and decorating the structure with Ag have both affected the sensitivity of the sensors. The pure Ni-Al LDH sensor has a response value of 6.8–250 ppm of ethanol. By increasing the specific surface area and creating LDH nanosheets, the response value can be increased up to 8.52. Also, the addition of Ag NP (Ni-Al LDH decorated with Ag NP) has increased the response up to 10.89. It was also observed that the response times in different modes have the least difference. At the end, the response mechanism was investigated based on the heterojunction theory.

Optimal monolayer WO3 nanosheets/TiO2 heterostructure and its photocatalytic performance under solar light

Highly efficient photocatalysts were formed via high-temperature calcination of monolayer WO3 prepared by a space-constrained method combined with commercial TiO2, and the optimal ratio of these components was explored. These synthetic catalysts were also characterized. During photocatalysis, the decomposition rate of MB achieved by the 1 mol% ML-WO3/TiO2 composite reached 98.05% within 10 min under solar light. Moreover, the photocatalytic efficiency of the composite catalyst did not decrease substantially after 5 cycles of operation. In addition, energy band position and active gene capture experiments revealed that the prepared photocatalysts conform to the transfer theory of Z-scheme heterojunctions.

Ethylene chlorotrifluoroethylene/hydrogel-based liquid-solid triboelectric nanogenerator driven self-powered MXene-based sensor system for marine environmental monitoring

Triboelectric nanogenerators (TENGs) have great application prospects in self-powered detection system. In this paper, a wave-driven liquid-solid TENG was developed to construct a self-powered sensing system for marine environmental monitoring. The TENG was constructed with ethylene chlorotrifluoroethylene (ECTFE) film and ionic hydrogel electrodes to harvest wave energy. The peak-to-peak value of open-circuit voltage and powered density from the TENG can reach up to 332 V and 1.85 W/m2, respectively. The self-powered MXene/TiO2/SnSe sensor driven by the TENG was prepared for SO2 gas detection, which has an excellent response (ΔU/Ua = 170 % @ 30 ppm) and is 14 times larger than that of the resistive sensor. The gas-sensing improvement mechanism was discussed in detail using ternary p-m-n heterojunction and density functional theory simulation. A self-powered marine environment monitoring system was developed to further demonstrate the application potential of the prepared TENG and sensor. Sensor signals from the self-powered system can be transmitted to smartphones and upper modules to monitor temperature, humidity, SO2 concentration, water surface height and other environmental factors in real time. In addition, the temperature and humidity error correction of the gas sensor was realized by integrating and processing the data of the sensor system with the back propagation neural network model.

A theoretical self-consistent method for the dislocation-based heterojunction

As an improvement of the conventional misfit dislocation theory, a theoretical self-consistent method is presented for a study of the heterojunction. Although the interaction between two adhesive crystals is still effectively described by the γ-energy function, however, the period appearing in the γ-energy function is no longer predetermined as is usually done but treated as a parameter needs to be determined in a self-consistent way. Because the period is actually the Burgers vector of the misfit dislocation, the self-consistent method is significant for the dislocation-based heterojunction theory. As an illustration, the theory is applied to the heterojunction of two-dimensional materials AlN and BN. It is found that for the AlN/BN heterojunction, the period determined from the minimum-energy principle is very close to the value given previously.

In-situ generated TiO2/α-Fe2O3 heterojunction arrays for batch manufacturing of conductometric acetone gas sensors

A highly sensitive and selective acetone gas sensor has been fabricated successfully by designing n-n heterojunction consisting of TiO2 nanorods and α-Fe2O3 branches. The structure and morphology of the TiO2/α-Fe2O3 nanocomposites were examined via X-ray diffraction and scanning electron microscopy, respectively. TiO2 nanorod (with the diameter about 42 nm) arrays were in-situ generated on the surface of Al2O3 ceramic tubes by a simple hydrothermal process. In addition, α-Fe2O3 branches grew on TiO2 stems successfully in the second hydrothermal process. These nanorod branches had a relatively uniform length, which could be tunable by changing the concentration of iron precursor. The gas sensing properties of the pristine and α-Fe2O3 branches-decorated TiO2 nanorods sensors with regard to acetone gas were investigated. The results indicated that the sensor based on TiO2/α-Fe2O3 heterostructures measured at 225 °C had a higher response of 21.9 toward 100 ppm of acetone gas which was about 9 times higher than pristine TiO2 nanorods sensor. The dominant mechanism for enhanced sensing properties were discussed in detail with the semiconductor depletion layer model and TiO2/α-Fe2O3 n–n heterojunction theory.

Carrier accumulation in organic heterojunctions controlled by polarization

The diode operation of OLEDs is characterized by the creation of a conductive space charge region in an insulating neutral dielectric region under the voltage application. It contrasts with the semiconductor diodes, where an insulating space charge region is created in a conductive charge neutral region. We proposed a heterojunction theory based on the quasi-conductor concept to give a unified view, which gave a modified Mott-Schottky equation for OLEDs (Org. Electron. 61, 10 (2018)). Based on this theory, we proposed a new mobility evaluation method using modulus spectroscopy and reported an in-situ simultaneous evaluation of mobilities and carrier densities of carrier transporting materials in a practical OLED configuration (Jpn.J.Appl.Phys. 59(7), 071005 (2020)). However, there were invalid handlings of the boundary conditions. We carefully reconsider the connection of the electric fields through the insulating and conductive regions based on the Dynamic Modulus Plot (DMP) analysis. It is proposed that the density of the accumulated carriers is governed by the fixed interface charges rather than the Activation of Localized Carrier Source (ALCS). Despite the errors in the previous derivation, the revised theory also gave luckily the same expression of mobility as a result, to ensure the validity of the quantitative estimation in the previous report.

Enhanced resistive acetone sensing by using hollow spherical composites prepared from MoO3 and In2O3.

Hollow sphere composites were synthesized by a template-free hydrothermal method from MoO3 and In2O3. The spheres have a typical size of 800 ± 50nm and were characterized by XRD, FESEM, TEM, XPS. Gas sensors based on samples with different Mo/In composite ratios were fabricated and their gas sensing properties were studied. The results show that a Mo:In ratio of 1:1 in the composite gives the highest response, typically at a working temperature of 250°C. The response increases to 38 when exposed to 100ppm acetone at 250°C. This is 13.6 times better than when using pure MoO3. The sensor shows improved selectivity, response, repeatability and long-term stability. Typical features include a large specific surface area, and high levels of chemisorbed oxygen and defective oxygen sites. The N-N heterojunction theory was used to explain the improvement of gas sensing performance. Graphical abstract Schematic presentation of MoO3 and In2O3 composites and response test graph for 100ppm acetone. The sensor based on this composite exhibits a very high response (38) to acetone at 250°C and very fast response time (2s).

Theory of carrier accumulation in organic heterojunctions

Abstract Even though organic light emitting diodes (OLED) have been achieving commercial successes, their device physics have been under debating. In this article, we develop a junction theory of organic heterojunctions, which describes the carrier accumulation dynamics in them. We derive an equation, which is the counterpart of the Mott-Schottky relation in the metal-semiconductor contact. We will show that the essence of the carrier transport control in organic devices is the control of the internal electric field and therefore the drift motion, whereas that in the semiconductors is the control of the carrier population by shifting the Fermi energy and therefore the diffusion motion. Tang's invention of organic electronic devices with layered ultrathin layers of small organic molecules was a new paradigm of controlling the carrier transport in polaron conductors rather than only a finding of the substitutes of Si semiconductors.

High Efficient Cu2O/TiO2 Nanocomposite Photocatalyst to Degrade Organic Polluant under Visible Light Irradiation

AbstractThe enhanced photocatalytic properties of Cu2O/TiO2 nanocomposites resulting from the synthesis of uniform p‐type Cu2O and n‐type TiO2 via wet chemical method were studied. The nanocomposite also had been well characterized and tested for its photocatalytic activity. These results indicate that the photocatalytic performance of the composites is clearly improved. The degradation rate of methylene blue solution (10 mg/L) increased to 93.67% in 45 minutes. The p‐n heterojunction theory was used to explain the enhanced photocatalytic properties. The photocatalytic performances of Cu2O/TiO2 nanocomposites and its photoreaction mechanism were proposed and discussed in this work.

Enhanced triethylamine sensing properties by fabricating Au@SnO2/α-Fe2O3 core-shell nanoneedles directly on alumina tubes

α-Fe2O3 nanoneedles were directly grown on the surface of Al2O3 tubes by a cost-effective hydrothermal method. In addition, the construction of Au nanoparticle-loaded SnO2/α-Fe2O3 core/shell heterostructure was fabricated by employing pulsed laser deposition (PLD) and DC-sputtering methods Gas sensors were fabricated from Au@SnO2/α-Fe2O3 core-shell nanoneedles on Al2O3 tubes, and their sensing properties have investigated for response to various target gases. The results indicated that the sensor based on Au@SnO2/α-Fe2O3 core-shell nanoneedles showed superior selectivity toward triethylamine (TEA) gas, giving a response of 39–100 ppm, which was higher than that of α-Fe2O3 nanoneedles and SnO2/α-Fe2O3 core-shell nanoneedles. Moreover, the sensor based on Au@SnO2/α-Fe2O3 core-shell nanoneedles showed an obvious better linearity (R = 0.9975) of sensing characteristics than of two other sensors. The enhanced sensing properties are discussed with the semiconductor depletion layer model along with the Schottky contact and N–N heterojunction theory.

Fabrication and characterization of alternating-current-driven ZnO-based ultraviolet light-emitting diodes

Alternating-current-driven ZnO-based light-emitting diodes (LEDs) with an Au/ZnMgO/ZnO/ZnMgO/GaN/In structure have been fabricated. Both polarity-controlled electroluminescence (EL) and ultraviolet emission were achieved when driven by an alternating current (AC) under a much lower root-mean-square voltage than that of conventional inorganic AC thin-film electroluminescent devices. This ZnO-based LED can be regarded as a series incorporating a metal-insulator-semiconductor diode and a p-i-n diode. The EL mechanisms were discussed in terms of the band diagrams and carrier transport behavior with reference to the semiconductor heterojunction theory.

Electroluminescence from ZnO-nanorod-based double heterostructured light-emitting diodes

Light-emitting diodes (LEDs) with MgZnO/ZnO/MgZnO double heterojunction structure have been fabricated and the room temperature electroluminescence (EL) spectra have been studied. With the help of double heterostructure, LEDs show better visible EL performance than that of LED with ordinary p-i-n structure. By replacing ZnO film with ZnO nanorod arrays in this double heterostructure, strong ultraviolet EL emission around 380 nm was achieved. The ZnO-nanorod-based double heterostructured light-emitting diode exhibits superior stability with an intensity degradation of less than 3% over 8 h. The EL mechanisms were discussed in terms of carrier confinement and carrier transport based on semiconductor heterojunction theory.

Structure and magneto-electrical properties of Fe-C films prepared by magnetron sputtering

Fe-doped amorphous Fe x C1−x granular films were prepared on n-Si (100) substrates by d.c. magnetron sputtering. The structural properties of Fe x C1−x films were investigated by X-ray diffraction (XRD), atomic force microscope (AFM) and Raman spectroscopy. The results show that the iron and carbon of as-deposited films are in amorphous state, and the Fe x C1−x films are diamond-like carbon (DLC) films. After doping iron into the DLC films, a smooth surface morphology of the Fe x C1−x films has been obtained with the surface roughness Ra of about 0.231 nm for x=18at%. The Fe x C1−x films have good soft magnetic properties with the coercivity of approximately 20 Oe. A high positive magnetoresistance (MR) up to 93% with x=1at% was observed in a Fe x C1−x granular film at 300 K. The resistance characteristic of Fe-C films is changed at about 230 K and the positive MR effect can be understood by the p-n heterojunction theory.

Magnetic properties and magnetoresistance of Co x C1−x granular films prepared by magnetron sputtering

Amorphous CoxC1−x granular films were prepared on n-Si(100) substrate by dc magnetron sputtering. The effects of Co concentration, film thickness and annealing temperature on the magnetic properties and magnetoresistance (MR) were investigated. After annealing at 500°C for 0.5 hour, the Co(002) peak of the CoxC1−x (x>2.5 at.%) films was observed, but cracks appeared in the films. Saturation magnetization Ms increased steadily with the increase of Co concentration from 2.5 at.% to 50 at.% and also increased with annealing temperature from room temperature to 400°C. The coercivity of CoxC1−x films was less than 180 Oe. The as-deposited Co2.5C97.5 granular films with 80 nm thickness showed a highly positive MR, up to 15.5% at a magnetic field of 0.8 T, observed at T=300 K when the external magnetic field was perpendicular to the film surface. With increasing film thickness and annealing temperature, the value of MR was found to decrease gradually and changed from positive to negative. The MR effect of the CoxC1−x granular films can be explained by p-n heterojunction theory and interface scattering effect.

Pin double-heterojunction thin-film solar cell p-layer assessment

The simplest realization of a pin double-heterojunction thin-film solar cell would consist of a lightly doped, moderate-bandgap absorber i-layer; a heavily doped, wide-bandgap n-layer window (cathode); and a heavily doped, wide-bandgap p-layer window (anode) in which the anode and cathode are electrically contacted by at least one transparent conductor. The focus herein is on p-layer interfacial assessment, which is accomplished using modern Schottky barrier and heterojunction theory and is directed to the analysis of p-windows for copper indium gallium diselenide (CIGS) and cadmium telluride (CdTe) thin-film solar cells. A p-type window layer serves as an electron reflector and also aids in the formation of an ohmic anode contact. Ohmic anode contacts are particularly difficult to form in CIGS and CdTe thin-film solar cells since these materials have very large ionization potentials, i.e., IP S = 5.65 (CIGS) and 5.78 V (CdTe) and significant interfacial screening, characterized by extremely small Schottky barrier interface parameters, i.e., S = 0.14 (CIGS) and 0.21 (CdTe). An ideal p-type window material would be heavily doped, p-type, and would have a wide bandgap, a large ionization potential, and a smaller charge neutrality level energy than that of the absorber layer.

Synthesis and enhanced ethanol sensing characteristics ofα-Fe2O3/SnO2 core–shell nanorods

α-Fe2O3/SnO2 core–shell nanorods are synthesized via a three-step process. X-ray diffraction (XRD),scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysesreveal that their diameters and lengths are respectively in the ranges 35–120 nm and0.35–1.2 µm, and the thickness of the shell composed of 3.5 nmSnO2 nanoparticles is about 10 nm. The core–shell nanostructures exhibit adramatic improvement in ethanol sensing characteristics compared to pureα-Fe2O3 nanorods. The sensor response is up to 19.6 under 10 ppm ethanol exposure at220 °C. Both the response time and the recovery time of the core–shell structures are less than 30 s. Basedon the space–charge layer model and semiconductor heterojunction theory, the small thickness of theSnO2 shell and the formation of heterojunctions contribute to the enhanced ethanolsensing characteristics. Our results demonstrate that one-dimensional metal oxidecore–shell nanostructures whose shell thickness is smaller than the Debye length arevery promising materials for fabricating gas sensors with good performances.

Transparent electronics: Schottky barrier and heterojunction considerations

Transparent electronics employs wide band gap semi-conductors which are transparent in the visible portion of the electromagnetic spectrum for the fabrication of electronic devices and circuits. Current and future transparent electronics applications require the use of wide band gap oxide semi-conductor interfaces as contacts and rectifiers, as well as for passivation and barrier-shaping layers. Modern Schottky barrier and heterojunction theory can be applied to the assessment of such interfaces, and is reviewed for this purpose from a charge transfer, energy band diagram perspective. Ideal interface formation theory is envisaged as originating from Fermi level mediated charge transfer giving rise to a macroscopic interfacial dipole, while non-ideal theory involves charge neutrality level mediated charge transfer giving rise to a microscopic interfacial dipole. This interface formation theory is applied to the problem of indium tin oxide (ITO) – zinc oxide and ITO – tin oxide interfaces, confirming their utility as injecting source-drain contacts in transparent thin-film transistors.

Optical and electrical characterization of the band structure of polyaniline nanofilms and polyaniline/silicon heterojunctions

We use near-IR absorption spectroscopy and electrical characterization to investigate the band structure of gamma-ray irradiated and non-irradiated polyaniline films and polyaniline/silicon heterojunctions. We have found that the band structure of the polyaniline/Si heterojunction can be modeled by taking in consideration the characteristic features of the polyaniline system, including HUMO and LUMO bands, interband transitions and doping. The silicon/polyaniline heterojunction was modeled combining the results of band structure calculations and simple heterojunction theory. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of Interface Charge on Electrical Properties ofZnO/Si Heterojunction

The capacitance–voltage and current–voltage properties of thesamples before and after annealing are investigated at 300 K and 195 K.It is found that the interface charge plays an important role for theheterojunction properties. After annealing, the samples exhibit typicaljunction properties. The heterojunction shows a built-in potential0.62 eV consistent with the theoretical result 0.67 eV. However, thesample exhibits more complex behaviour before annealing. Theexperimental results can be explained by heterojunction theory whenintroducing interface charge, which suggests that the annealing canreduce interface charge and can improve the junction properties of thesamples.

Amorphous germanium contacts on germanium detectors

The rectification properties of sputtered amorphous germanium contacts made under a variety of conditions on germanium detectors have been measured as a function of temperature and electric field. The leakage current increased as a function of voltage above depletion voltage. The rectifying amorphous–crystalline heterojunction theory derived by Döhler and Brodsky describes the behavior well. Values of barrier height were measured to be ∼0.30–0.35 eV and the density of states was found to be N F∼10 18 eV −1 cm −3.