Conduction Band Energy Research Articles

A detailed comparative performance analysis of the Transition Metal Di-chalcogenides (TMDs) based strain sensors through experimental realisations and first principle calculations

To date, most reports on the development of different Transition Metal Di-chalcogenide (TMDs) based strain sensors emphasize the qualitative analysis of the transduction mechanism of fabricated sensor devices based on a specific TMD. However, to the best of the author's knowledge, no reports are available on the systematic comparative performance analysis of different TMDs for strain sensing applications that correlate the sensing performance with electronic properties of individual TMDs. This limits the design-level understanding of TMD-based strain sensors and their performance optimization. In this context, this work demonstrates the fabrication and electrical characterization of strain sensors based on five TMDs (MoS2, MoSe2, SnSe2, NiSe2, and SnS2). Furthermore, the electronic properties of the individual TMDs are analysed using Density Functional Theory (DFT) based first-principle calculations and correlated with the experimentally observed sensing performance under applied strain. The strain sensors are modelled as the percolating networks of TMD nano-flakes, and the transduction mechanism is assessed as the modulations in the inter-flake quantum mechanical tunneling components and thereby overall conductivity of the network with applied strain. Subsequently, it has been demonstrated that the response of individual percolating networks of TMDs depends on the carrier concentrations, energy bandgap, and Density of states (DOS) for a specific applied strain. The influence of these properties on the transduction mechanism is theoretically analysed, leading to a clear casual correlation between the experimental observed Gauge factors and the theoretically calculated energy bandgap and conduction band DOS/effective masses. The higher gauge factors measured experimentally are for selenides-based TMDs, which also collaborates with the simulations studies suggesting that the materials with lower bandgap and higher DOS are best suited for physical sensing applications.

Efficiency and Stability Analysis of 2D/3D Perovskite Solar Cells Using Machine Learning

A dataset containing 599 data points from 146 publications on 2D/3D perovskite solar cells is analyzed using machine learning. The predictive models are developed for power conversion efficiency (PCE) using eXtreme Gradient Boosting regression, random forest regression and artificial neural networks while association rule mining is used to analyze the stability data to identify the descriptors leading to high stability 2D/3D cells. A predictive model is also developed for the bandgap to predict the missing values in the dataset for the use in PCE predictions. Models for both bandgap and PCE predictions are quite successful. The thickness of inorganic layer (n), radius of anion (R x ), and 2D cation (R m) are found to be the most important descriptors for bandgap predictions; n and R m, together with the bandgap, are found to be deterministic for PCE in regular cells while the bandgap, n, and conduction band energy of hole transport layer are the most influential descriptors in inverted structures. Association rule mining analysis for the stability indicates that the cells with layered perovskite structures are more stable while the 2D and 3D cations leading to the most stable cells are found to be butylammonium and formamidinium‐Cs mixed cation respectively.

Solar cells efficiency enhancement using multilevel selective energy contacts (SECs)

High-energy incoming photons can be absorbed and concluded to generate Hot Carriers. In normal solar cells, these carriers are scattered by electron–electron and electron-lattice mechanisms and rapidly lose extra energy and approach to conduction band energy edge. This event plus other loss mechanisms cause that the efficiency of the solar cells to be limited to 33% theoretically. If one makes the possibility for carriers that can be extracted rapidly, before scattering and releasing extra energy to the lattice, the efficiency of solar cells is enhanced considerably. This type of solar cell is named hot carrier solar cells (HCSCs). To this end and improvement the conversion efficiency, multilevel energy selective contacts (ESCs) as a new concept and new mechanism in solar cells are used. In the other words, several appropriate energy levels as carrier extraction contacts in the conduction band are introduced. Here, we use multilevel ESCs, and based on our simulation it is shown that the maximum efficiency of 75% is achievable for low bandgap materials. For a typical material such as Si, the maximum efficiency is increased to 60% using ten ESCs.

Construction of CdSe-AuPd quantum dot 0D/0D hybrid photocatalysts: charge transfer dynamic study with electrochemical analysis for improved photocatalytic activity.

The integration of semiconductor quantum dots and noble metal nanoparticles can efficiently couple numerous effects corresponding to the individual domains of the hybrid system for a variety of applications. Herein, we establish a direct correlation between the electronic band structure and optical band gap of monometallic and bimetallic alloy nanoparticle decorated CdSe quantum dots, which in turn regulate the charge shuttling dynamics in a quantum dot hybrid (QDH) system. Directly coupled Au, Pd, AuPd, and CdSe QDHs were prepared via a simple fabrication technique. The photoluminescence intensity of the QDHs was quenched compared to that of CdSe quantum dots with a maximumally diminished CdSe-AuPd system. Broadening of the absorbance peak along with a blue shift for QDHs confirm the interaction of the energy levels of the QDs and metal domains. AuPd decorated CdSe QDs demonstrate enhanced photocatalytic activity compared to their monometallic counterparts, which has made them interesting catalysts reported for the first time. Lifetime decay measurements, which isolated the individual charge-transfer steps, showed that a maximum amount of photoexcitons can be separated by bimetallic alloy decoration compared to monometallic ones. Cyclic voltammetry results offer insight into the change in the conduction band edge energy position for both monometallic and bimetallic incorporating semiconductor hybrid systems. Our findings reveal that photoexcited semiconductor quantum dots undergo charge equilibration when the QDs are in contact with metallic domains, influencing the shifting of the conduction band energy level of the hybrid to a more negative potential, and this is a maximum for the CdSe-AuPd hybrid, resulting in the best photocatalytic activity. Shuttling of electrons around the conduction band of CdSe and the Fermi level of the metallic domains is the main deciding factor for an efficient photocatalyst hybrid system.

Optimization of interfacial characteristics of antimony sulfide selenide solar cells with double electron transport layer structure

Antimony sulfide selenide thin film solar cells have drawn great interest in the field of photovoltaic due to their advantages of simple preparation method, abundant raw materials, non-toxic and stable photoelectric properties. After the development in recent years, the photoelectric conversion efficiency of antimony sulfide selenide solar cells has exceeded 10%, which has great development potential. In this work, the carrier recombination near n/i interface in antimony sulfide selenide solar cells is studied. It is found that the characteristics of the n/i interface are affected by the interfacial electron mobility and energy band structure. The improvement of the interface electron mobility can make the electrons more effectively transferred to the electron transport layer, and realize the effective improvement of the short circuit current density and fill factor of the device. Moreover, the introduction of ZnO/Zn<sub>1–<i>x</i></sub>Mg<sub><i>x</i></sub>O double electron transport layer structure can further optimize the performance of antimony sulfide selenide solar cells. The change of Zn<sub>1–<i>x</i></sub>Mg<sub><i>x</i></sub>O energy level position can adjust the energy level distribution of the interface and light absorption layer simultaneously. When the conduction band energy level of Zn<sub>1–<i>x</i></sub>Mg<sub><i>x</i></sub>O is –4.2 eV and the corresponding Mg content is 20%, the effect of restraining the carrier recombination is the most obvious, and the antimony sulfide selenide solar cell also obtains the best device performance. Finally, under the ideal condition of removing the defect state, the antimony sulfide selenide solar cells with 600 nm in thickness can achieve 20.77% theoretical photoelectric conversion efficiency. The research results provide theoretical and technical support for further optimizing and developing the antimony sulfide selenide solar cells.

Quantum anomalous Hall state with Chern number C=2 in wurtzite quantum wells

Quantum anomalous Hall (QAH) effect in topological insulators (TIs) is known to have involved energy bands with Chern numbers $C=\ifmmode\pm\else\textpm\fi{}1$. The QAH edge current would be electrically more accessible if one suppressed the edge-channel resistance, $h/C{e}^{2}$. That is why topological states $|C|>1$ are preferable for connecting them to an external circuit in device applications. The search for topological systems with a high Chern number is ongoing. This paper points to a semiconductor material system well developed for optoelectronic applications----III-nitrides. Polarization field in a compressed InN quantum well (QW) has two effects: it inverts conduction and valence energy bands, thus creating a prerequisite for topological insulator (TI), and generates Rashba spin splitting in conduction and valence bands. We calculate topological indexes in GaN/InN/GaN QW. In the QAH setting, value $|C|>1$ follows from the phase structure of interband coupling in III--V materials, providing Rashba and Zeeman fields act simultaneously. In nonmagnetic samples, explicitly calculated topological index ${Z}_{2}=1$ manifests the quantum spin Hall state.

Influence of back barrier layer thickness on device performance of AlGaN/GaN MOS-HEMT

In this work, we have performed the influence of back barrier layer thickness variation on AlGaN/GaN Metal Oxide Semiconductor High Electron Mobility Transistor (MOS-HEMT) device with 0.5 µm Schottky gate length. The AlGaN back barrier layer presented increases the conduction band with respect to GaN channel layer so that more no of electron confinement into the GaN channel layer and improve the high-frequency performance. The effect of the back-barrier layer thickness is performed by using 2-D TCAD Atlas Silvaco numerical simulation tool by taking Hydrodynamic mobility model. Due to a large amount of two-dimensional electron gas (2-DEG) density at the AlGaN/GaN heterointerface of the MOS-HEMT device higher drain current density is obtained. The 2-D simulation is carried out with a variation of back barrier layer thickness for various device parameter such as transfer characteristics (Id-Vg), drain current with a drain voltage (Id-Vd), transconductance (gm), drain induced barrier lowering (DIBL), conduction band energy and electron concentration into the channel. In this simulation, we have also performed the RF performance like a gate to source capacitance (Cgs) and current gain cut-off frequency of AlGaN/GaN MOS-HEMT device. The results obtained by variation of AlGaN back barrier layer thickness can be a better solution in future analog and RF device application. Copyright © 2018 VBRI Press.

Soluble Complexes of Cobalt Oxide Fragments Bring the Unique CO2 Photoreduction Activity of a Bulk Material into the Flexible Domain of Molecular Science.

The deposition of metal oxides is essential to the fabrication of numerous multicomponent solid-state devices and catalysts. However, the reproducible formation of homogeneous metal oxide films or of nanoparticle dispersions at solid interfaces remains an ongoing challenge. Here we report that molecular hexaniobate cluster anion complexes of structurally and electronically distinct fragments of cubic-spinel and monoclinic Co3O4 can serve as tractable yet well-defined functional analogues of bulk cobalt oxide. Notably, the energies of the highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO) of the molecular complexes, 1, closely match the valence- and conduction-band (VB and CB) energies of the parent bulk oxides. Use of 1 as a molecular analogue of the parent oxides is demonstrated by its remarkably simple deployment as a cocatalyst for direct Z-scheme reduction of CO2 by solar light and water. Namely, evaporation of an aqueous solution of 1 on TiO2-coated fluorinated tin oxide windows (TiO2/FTO), immersion in wet acetonitrile, and irradiation by simulated solar light under an atmosphere of CO2 give H2, CO, and CH4 in ratios nearly identical to those obtained using 20 nm spinel-Co3O4 nanocrystals, but 15 times more rapidly on a Co basis and more rapidly overall than other reported systems. Detailed investigation of the photocatalytic properties of 1 on TiO2/FTO includes confirmation of a direct Z-scheme charge-carrier migration pathway by in situ irradiated X-ray photoelectron spectroscopy. More generally, the findings point to a potentially important new role for coordination chemistry that bridges the conceptual divide between molecular and solid-state science.

Tensile-strained self-assembly of InGaAs on InAs(111)A

We have determined a reproducible set of growth conditions for the self-assembly of tensile-strained In1−xGaxAs quantum dot (QD) nanostructures on (111)A surfaces. During molecular beam epitaxy, In1−xGaxAs islands form spontaneously on InAs(111)A when the Ga content x≥50%. We analyze the structure and composition of InGaAs/InAs(111) samples using atomic force microscopy, transmission electron microscopy, and electron energy loss spectroscopy. We demonstrate control over the size and areal density of the islands as a function of In1−xGaxAs coverage, In1−xGaxAs composition, and substrate temperature. We calculated the conduction and valence band energy values for these QDs in an InAs matrix. This work supports the efforts to establish InAs(111)A as a platform for future incorporation with other (111)-oriented materials from the 6.1 Å family of semiconductors.

Novel two-dimensional beta-XTe (X = Ge, Sn, Pb) as promising room-temperature thermoelectrics.

In this paper, we designed novel low-symmetry two-dimensional (2D) structures based on conventional XTe (X = Ge, Sn, Pb) thermoelectrics with large average atomic mass. The first-principles calculations combined with Boltzmann transport theory show that the beta-XTe exhibit good stability, high electron carrier mobility, and ultralow ΚL. The subsequent analyses show that the ultralow ΚL stems from the coexistence of resonant bonding, weak bonding, and lone-pair electrons in beta-XTe, which leads to large anharmonicities. On the other hand, the lowest energy conduction band of beta-GeTe and beta-SnTe show the convergence of the low-lying Ʃ band, which is the source of the high-power factor in the two systems. The calculated maximum ZT of beta-XTe (X = Ge, Sn, Pb) are 3.08, 1.60, and 0.57 at 300K, respectively, which is significantly greater than that of the previously reported high-symmetry 2D alpha-XTe and the commercial thermoelectrics. We hope that this work can provide important guidance for the development of thermoelectric materials.

Weak-Field Hall Resistivity and Spin-Valley Flavor Symmetry Breaking in Magic-Angle Twisted Bilayer Graphene.

Near a magic twist angle, the lowest energy conduction and valence bands of bilayer graphene moiré superlattices become extremely narrow. The band dispersion that remains is sensitive to the moiré's strain pattern, nonlocal tunneling between layers, and filling-factor-dependent Hartree and exchange band renormalizations. In this Letter, we analyze the influence of these band-structure details on the pattern of flavor symmetry breaking observed in this narrow band system and on the associated pattern of Fermi surface reconstructions revealed by weak-field Hall and Shubnikov-de Haas magnetotransport measurements.

Π-Shape Silicon Window for Controlling OFF-Current in Junctionless SOI MOSFET

In this paper a modified junctionless transistor is proposed. The aim of the novel structure is controlling off-current using π-shape silicon window in the buried oxide under the source and the channel regions. The π-shape window changes the potential profile in the channel region in which the conduction band energy get away from the body Fermi energy and rebuild an electrostatic potential. Beside the significant reduced off-current, on current has acceptable value in the novel Silicon Region Junctionless MOSFET (SR-JMOSFET) than Conventional Junctionless MOSFET (C-JMOSFET). Moreover, replacing silicon material instead of silicon dioxide in the buried oxide causes reduced maximum temperature in the channel region. In this situation the heat could transfer to the π-shape silicon window and the temperature reduces in the active region, significantly. The simulation with the two-dimensional ATLAS simulator shows that short channel effects such as subthreshold and DIBL are controlled effectively in the SR-JMOSFET. Also, the optimum values of length and thickness of the π-shape window are defined to obtain the best behavior of the device.

A simulation study of the impact of traps in the GaN substrate on the electrical characteristics of an AlGaN/GaN HEMT with a thin channel layer

Gallium nitride (GaN) substrates are promising candidates for GaN high-electron-mobility transistors (HEMTs) because of their epitaxial layer growth with low defect density. We perform device simulations to study the influence of substrate acceptor traps in GaN HEMTs on semiinsulating GaN substrates with a thin (0.02 μm) channel layer. When the trap concentration in the GaN substrate increases at a constant channel trap concentration of 1.0 × 1015 cm−3, the drain leakage current decreases but the transient response worsens. These phenomena result from an increase of the conduction-band energy due to ionized acceptor traps with negative charge. For GaN buffer structures, we obtain moderate suppression of the drain leakage current (1.7 × 10−9 A/mm) and a good transient response (with a normalized drain current of 0.86 at 1 ms for the transient response from the off- to on-state condition) at a substrate trap concentration of 5.0 × 1015 cm−3. The aluminum gallium nitride (AlGaN) back-barrier structure is highly effective for suppressing the drain leakage current at a low trap concentration of 1.0 × 1015 cm−3 in GaN substrates. Although its maximum drain current is decreased, this structure exhibits a low drain leakage current (4.7 × 10−11 A/mm) and a high normalized drain current (0.95) in the transient response. Increasing the Al content in the barriers of the GaN HEMT structure increases its maximum drain current to 1.2 A/mm, whereas the drain leakage current and transient response are well maintained. Moreover, the traps in the GaN substrate affect the low-frequency S21, which is important for the linearity of power amplifiers, and the characteristics of S21 are similar to those of the transient responses.

Synthesizability and Properties of Metastable Semiconductors for Solar Energy Conversion

Metastable semiconductors in many chemical systems have been discovered that have desirable semiconductor properties for driving fuel-producing redox reactions from water and sunlight, such as strong visible light absorption, optimal band-edge energies, extreme defect tolerance, and enhanced carrier mobilities and charge separation. These properties are intimately related to their metastable nature, i.e., their crystalline structures and compositions lead to thermodynamic instabilities. New results will be presented that demonstrate flux-mediated synthetic routes to new metastable n-type Sn(II)-titanates and p-type Cu(I)-niobates. These semiconductors possess some the smallest known visible-light band gaps that also maintain suitable conduction and valence band energies for the photocatalytic reduction and oxidation of water to molecular hydrogen and oxygen, respectively. Their electronic structures also show advantageous features for solar energy conversion, such as small bandgaps, high band dispersion and defect tolerance. These characteristics are intimately related to their metastability and kinetic stabilization, which can be achieved via solid solutions that inhibit phase segregation. Thus, the solid-solution semiconductors Cu5(Ta1-x Nb x )11O30 and (Ba1-x Sn x )(Zr1-yTi y )O3 have been prepared using flux-mediated synthesis methods. Their band gaps, band edge energies and photocatalytic activities for the production of molecular hydrogen and molecular oxygen are found to be a sensitive function of their chemical compositions.

Enhancing the Luminous Efficiency of Ultraviolet Light Emitting Diodes By Adjusting the Al Composition of Pre-Well Superlattice

The lower luminous efficiency is a critical issue for ultraviolet light-emitting diodes (UV-LEDs) owing to the poor carrier injection efficiency and high dislocation density. Here, we can improve the luminous efficiency in two avenues by adjusting the Al composition of the InGaN/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> N pre-well superlattice. First, due to the strain-induced piezoelectric and intrinsic spontaneous polarization, a large number of electrons gather at the InGaN/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> N interface, which improves the electron concentration of the pre-well superlattice and lowers the conduction band energy of the first quantum barrier layer (FQB), thus enhancing the electron injection efficiency. Second, the pre-well superlattice can act as a hole blocking layer to prevent holes from leaking into the n-type layer and confine them in the quantum well layer. As the Al composition increases, the hole blocking effect of the pre-well superlattice is strengthened. However, higher Al composition decreases the lattice quality, which makes it possible for carrier loss through defect-related non-radiative recombination. Finally, the output power of the samples with 5% Al composition in the pre-well superlattice is 5.9% and 102.5% higher than that of the samples with 3% and 7% Al composition, respectively.

Theory of Charge Transfer Reaction Process at the Sensitizers Molecule Dye N3 Contact with MgO Semiconductor

A theoretical charge transport rate approach has taken to study the charge transfer properties in non-homogeneous N3-MgO systems. It develops at the fully quantum transition theory by means of transition energy, potential, driving energy and coupling constant. It is obtained that transition energy is determined by the donor acceptor scenario, dependent on the radii of N3 and MgO, dielectric constant and refractive index of solvents. The transition energy of charge carriers increased with increased dielectric constant and decreased refractive index of solvents. Transition energy of N3-MgO system reach to top with methanol (0.582 ev) and has minimum with Chlorobenzene (0.104eV). Dependences of the driving energy versus chemical potential of N3 dye and conduction band of semiconductor with potential barrier, the charge transfer rate are increased with decreased driving force of system. It is established that increased coupling constant factor reduces to increased charge transfer rate.

Revealing the Intrinsic Nature of the Synergistic Effect Caused by the Formation of Heterojunctions in Cu-Cu2O/rGO-NH2 Nanomaterials in the Catalysis of Selective Aerobic Oxidation of Benzyl Alcohol.

Ternary nanomaterials Cu-Cu2O/rGO-NH2 (rGO = reduced graphene oxide) exhibited a synergistic effect in the quantitative catalysis of selective aerobic oxidation of benzyl alcohol. The synergistic effect is attributed to the heterojunctions among the three components and in intrinsic nature, the formation of the heterojunctions lowered the conduction band (CB) energy level and raised the valence band (VB) energy level of the main catalyst Cu2O, which eases electron transfer from the catalyst to O2 in its activation and from the substrate to the catalyst in the oxidation, respectively.

Mechanistic study of Cr (VI) removal by modified alginate/GO composite via synergistic adsorption and photocatalytic reduction

A novel composite material was prepared by blending graphene oxide into polyethyleneimine grafted sodium alginate. The synthesized material was investigated as adsorbent and photocatalyst for the removal of hexavalent chromium (Cr (VI)) from aqueous solutions. The composite material has shown remarkable removal efficiency for Cr (VI) in high initial concentration solutions as the removal rate reached 86.16% and 99.92% for adsorption and photoreduction, respectively. We discovered experimentally that the adsorption was dominated via electrostatic interaction while the blending of GO could contribute in stimulating electrons for the photoreduction process. Moreover, the photoreduction can alter the surface charge of chromium species, thus electrostatic repulsion could regenerating the active sites of composite spontaneously. The conduction band energy was calculated as −2.04 eV, which proved that blending GO can narrow the bandgap of the composite material, thus enhance the light response and the photoreduction ability towards Cr (VI).

Remarkable improvement in hydrogen storage capabilities of graphitic carbon nitride nanosheets under selected transition metal embedding: A DFT study

The adsorption performance of hydrogen molecules over the transition metals (TM = Os, Ru, and Fe)-embedded graphitic carbon nitride (gCN) and also the binding energy of these TM elements over the gCN are investigated using DFT computations. The obtained results showthat the interaction energy between Os-embedded gCN and H2 molecule (with Eads of −2.452 eV) is superior than those of the other reported adsorbents. Based on these results, it is inferred that the maximum storage number of H2 molecules adsorbed over the TM–embedded gCN are 6 hydrogen molecules. The results reveal that with adsorption of H2 molecules over the gCN, conduction band and valence band energy levels have crossed each other close to the Fermi level EF, thus the semi-conductive behavior of these systems is converted to a conductive state. Finally, it is concluded that the Os–modified gCN is suitable for storaging of H2 molecules.

First-principles calculations of electronic and optical properties of A and B site substituted BaTiO3

In order to better understand the doping process at A site and B site, we have researched the X (Sr2+, Zn2+, Nd3+, Ni2+, Zr4+, Hf4+) doping of barium titanate (BaTiO3) by using the CASTEP. This work computed the distinct optical, structural, and electrical characteristics before and after doping and analyzed them in depth. After doping, the addition of Sr, Zn and Nd causes the low energy level and conduction band of the system to shift to the direction of low energy, and narrows the band gap. While after the B-site ion replaces Ti, the width and intensity of the conduction band state density also widen and increase. What's more, after doping, the cubic phase rises to the tetragonal phase, increasing the quality of BaTiO3. The optical spectra have dramatically different energy characteristics, making doped BaTiO3 promising for applications in a variety of optical and optoelectronic installations.