Band Edge Energy Research Articles

Synergetic surface defect passivation towards efficient and stable inorganic perovskite solar cells

Metal halide inorganic perovskites with excellent thermal stability are holding promise for practical application of perovskite solar cells (PSCs). However, ionic defects, especially iodine vacancies or undercoordinated lead, from the perovskite film surface restrain the obtainment of maximum open circuit voltage as well as long term stability of devices. In this regard, reducing the detrimental defects is of priority to further enhance the performance of inorganic PSCs. Herein, we employed an organic amine compound, specifically diethyldithiocarbamic acid diethylamine (DADA), to passivate the ionic defects on the surface of CsPbIxBr3-x inorganic perovskite film. It is found that the electron-rich sulfur atoms from DADA molecule react with undercoordinated Pb in perovskite, while diethylamine cation effectively forms hydrogen bond with halide ion, further strengthening the passivation effect. Such synergetic passivation, together with the improved film morphology, significantly reduces the defect density and prolongs the lifetime of charge carriers within the perovskite film. Moreover, DADA treatment induces an upward shift of the energy band edge of the perovskite layer, which may facilitate hole extraction at the interface. These combination raises the solar cell efficiency from 18.56% to 20.06% under 100 mW cm−2 illumination. More importantly, the mitigation of ionic defects via DADA passivation dramatically improve the devices ambient stability by retaining the PCE of nearly 90% over 800 h in a 30–40% relative humidity environment.

A Transparent, High-Performance, and Stable Sb2 S3 Photoanode Enabled by Heterojunction Engineering with Conjugated Polycarbazole Frameworks for Unbiased Photoelectrochemical Overall Water Splitting Devices.

Developing low-cost, high-performance, and durable photoanodes is essential in solar-driven photoelectrochemical (PEC) energy conversion. Sb2 S3 is a low-bandgap (≈1.7eV) n-type semiconductor with a maximum theoretical solar conversion efficiency of ≈28% for PEC water splitting. However, bulk Sb2 S3 exhibits opaque characteristics and suffers from severe photocorrosion, and thus the use of Sb2 S3 as a photoanode material remains underexploited. This study describes the design and fabrication of a transparent Sb2 S3 -based photoanode by conformably depositing a thin layer of conjugated polycarbazole frameworks (CPF-TCzB) onto the Sb2 S3 film. This structural design creates a type-II heterojunction between the CPF-TCzB and the Sb2 S3 with a suitable band-edge energy offset, thereby, greatly enhancing the charge separation efficiency. The CPF-TCzB/Sb2 S3 hybrid photoanode exhibits a remarkable photocurrent density of 10.1mAcm-2 at 1.23V vs reversible hydrogen electrode. Moreover, the thin CPF-TCzB overlayer effectively inhibits photocorrosion of the Sb2 S3 and enables long-term operation for at least 100h with ≈10% loss in photocurrent density. Furthermore, a standalone unbiased PEC tandem device comprising a CPF-TCzB/Sb2 S3 photoanode and a back-illuminated Si photocathode can achieve a record solar-to-hydrogen conversion efficiency of 5.21%, representing the most efficient PEC water splitting device of its kind.

Nanoscale observation of subgap excitations in β-Si3N4 with a high refractive index using low-voltage monochromated STEM: a new approach to analyze the physical properties of defects in dielectric materials

We observed nanoscale distribution of subgap excitations induced by Ga-ion beam processing in β-Si3N4 via electron energy-loss spectroscopy performed using a monochromated (0.1 eV) and aberration-corrected scanning transmission electron microscope. A sufficiently low operating voltage (30 kV) was selected to suppress background caused by Cerenkov loss in β-Si3N4 with a high refractive index. By further combining crystallinity, composition, and bandgap measurements, we found that defects excited at the band edge (6 eV) and lower energies (3 eV) exhibit different dependence trends with respect to crystallinity. The proposed technique was verified to effectively distinguish between various amorphous materials.

In-plane optical and electrical anisotropy in low-symmetry layered GeS microribbons

Layered group-IV monochalcogenides, including GeS, GeSe, SnS, and SnSe, garner attention because of their anisotropic structures and properties. Here, we report on the growth of GeS microribbons via chemical vapor transport (CVT), which affords each of them with a low-symmetry orthorhombic structure and anisotropic optical and electronic properties. The single-crystalline nature of the GeS microribbon, which has a typical thickness of ~30 nm, is confirmed. Polarized Raman spectra reveal angle-dependent intensities that are attributed to the anisotropic layered structure of GeS microribbons. The photoluminescence (PL) spectra reveal a peak at ~1.66 eV. The angle-dependent PL and anisotropic absorption spectroscopy results provide evidence for a distinct anisotropic optical transition near the energy band edges; this phenomenon is also predicted by our density functional theory (DFT)-based calculations. Strong in-plane direct-current transport anisotropy is observed under dark and white illumination by using back-gate cross-shaped field effect transistors (CSFETs) fabricated with the GeS microribbon; significant gate-tunable conductivity is also confirmed. The strong anisotropy is further confirmed by the DFT-calculated effective mass ratio. Our findings not only support the application of GeS microribbons in anisotropic photoelectronic transistors but also provide more possibilities for other functional device applications.

Valence band structure of biaxial strained Germanium for hole transport calculation

A comprehensive study of hole mobility in Germanium material dependence on strained substrate orientations and carrier transport directions was performed in this paper. The heavy hole, light hole, and spin–orbit bands are included in the calculation of valence band structure. The biaxial strain induced valence band edge energy splitting and effective mass change were evaluated as well as their impacts on the hole mobility in Germanium were analysed. The results show that the optimum strained substrate orientation and carrier transport direction configuration are biaxial compressive strained on the (110) oriented substrate and in-plane as a hole transport direction, the configuration of which has the maximum hole mobility under the same strained level in comparison with others. The reason for high hole mobility is also explored. The current research is helpful to design the high performances p-channel MOSFETs in the strained Germanium system. The hole mobility of Germanium versus strain level under different biaxial strained substrate and transport directions configurations

Speeding up tight binding calculations using zone-folding methods

Tight binding models are widely used in large scale electronic structure calculations of nanostructures. Their atomistic nature makes them flexible, but also means the computational cost increases rapidly with system size. The large number of calculations required to design nanostructures makes computational efficiency desirable. We have developed a method to increase computational speed while retaining most of its accuracy. The method is based on the use of supercells and zone folding combined with a truncation of the Hamiltonians to only include states close to the band-edges. We apply the method to model the band edge energies of a GaAs/AlAs quantum well grown along the [110]-directions with 3D and 2D periodic boundary conditions as well as the density of states and dielectric function of the quantum well. We typically find a speed-up of ten times with only a small loss of accuracy of the calculation result.

A DFT+U approach: Superior charge transfer characteristics and optoelectronic properties of GQD@TiO2 rutile (110) surface for improved hydrogen evolution

The understanding on complex electronic structure and charge transfer characteristics is crucial in the development of heterostructure-based photocatalyst such as graphene quantum dots (GQD) and titanium dioxide (TiO2). Simulation studies will be useful in gaining valuable insight on the complex system at atomistic level. Herein, for the first time, a theoretically designed heterogeneous GQD modified TiO2 (110) interface model using Hubbard's modified first-principles density functional theory (DFT+U) is presented. The structural properties were simulated with Perdew–Burke–Ernzerhof assisted generalized gradient approximation (GGA+PBE) and optoelectronic properties with Hubbard's modified (GGA+U) exchange correlation functional. The addition of GQD reduces the bandgap energy of the TiO2 rutile (110) surface from 2.95 eV to 1.86 eV, thereby improving the visible light response as it reduces the electron transition energy. Charge density difference map and Mulliken population analysis demonstrate frequent transfer of charges from GQD to the TiO2 surface, resulting in reduction of charge recombination rate. Moreover, the energy band edge estimation confirms a suitable band edge position in accordance with the redox potential of water. The collective effect of the heightened absorption of visible light and effective charge separation led to a significant photocatalytic performance of the hybrid photocatalyst.

A proof of concept of the bulk photovoltaic effect in non-uniformly strained silicon

We numerically investigate non-uniformly strained Si-based systems to demonstrate that when a well focused laser beam locally excites the sample, the lattice distortion, impacting the band edge profile, causes a spatially dependent photovoltaic effect. It follows that, scanning the sample surface with the pump spot, a photovoltage signal can be acquired and used to quantitatively map the non-uniform strain field. To provide numerical evidence in this direction, we combine mechanical simulations with deformation potential theory to estimate the band edge energy landscape of a Si lattice strained by an array of SiN stripes fabricated on the top surface. These data are then used to simulate the voltage signal obtained scanning the sample surface with a normal incident pump beam. Our analysis suggests that strain deformations as small as 0.1% can trigger at room temperature robust photovoltaic signals. These results allow us to envision the development of a fast, cost-effective, and non-destructive setup, which leverages on the bulk-photovoltaic effect to image the lattice deformation in semiconductor crystals.

Smart PdH@MnO2 Yolk-Shell Nanostructures for Spatiotemporally Synchronous Targeted Hydrogen Delivery and Oxygen-Elevated Phototherapy of Melanoma.

Hydrogen therapy, an emerging therapeutic strategy, has recently attracted much attention in anticancer medicine. Evidence suggests that hydrogen (H2) can selectively reduce intratumoral overexpressed hydroxyl radicals (•OH) to break the redox homeostasis and thereby lead to redox stress and cell damage. However, the inability to achieve stable hydrogen storage and efficient hydrogen delivery hinders the development of hydrogen therapy. Furthermore, oxygen (O2) deficiency in the tumor microenvironment (TME) and the electron-hole separation inefficiency in photosensitizers have severely limited the efficacy of photodynamic therapy (PDT). Herein, a smart PdH@MnO2/Ce6@HA (PHMCH) yolk-shell nanoplatform is designed to surmount these challenges. PdH tetrahedrons combine stable hydrogen storage and high photothermal conversion efficiency of palladium (Pd) nanomaterials with near-infrared-controlled hydrogen release. Subsequently, the narrow bandgap semiconductor manganese dioxide (MnO2) and the photosensitizer chlorin e6 (Ce6) are introduced into the PHMCH nanoplatform. Upon irradiation, the staggered energy band edges in heterogeneous materials composed of MnO2 and Ce6 can efficiently facilitate electron-hole separation for increasing singlet oxygen (1O2). Moreover, MnO2 nanoshells generate O2 in TME for ameliorating hypoxia and further improving O2-dependent PDT. Finally, the hyaluronic acid-modified PHMCH nanoplatform shows negligible cytotoxicity and selectively targets CD44-overexpressing melanoma cells. The synergistic antitumor performance of the H2-mediated gas therapy combined with photothermal and enhanced PDT can explore more possibilities for the design of gas-mediated cancer therapy.

Temperature effect on charge-state transition levels of defects in semiconductors

Defects are crucial in determining the overall physical properties of semiconductors. Generally, the charge-state transition level (TEL), one of the key physical quantities that determines the dopability of defects in semiconductors, is temperature dependent. However, little is known about the temperature dependence of TEL, and, as a result, almost all existing defect theories in semiconductors are built on a temperature-independent approximation. In this article, by deriving the basic formulas for temperature-dependent TEL, we have established two fundamental rules for the temperature dependence of TEL in semiconductors. Based on these rules, surprisingly, it is found that the temperature dependences of TEL for different defects are rather diverse: it can become shallower, deeper, or stay unchanged. This defect-specific behavior is mainly determined by the synergistic or opposing effects between free energy corrections (determined by the local volume change around the defect during a charge-state transition) and band edge changes (which differ for different semiconductors). These basic formulas and rules, confirmed by a large number of state-of-the-art temperature-dependent defect calculations in GaN, may potentially be widely adopted as guidelines for understanding or optimizing doping behaviors in semiconductors at finite temperatures.

Pathways for Electron Transfer at MgO-Water Interfaces from Ab Initio Molecular Dynamics.

The nature of electron transfer across metal oxide-water interfaces depends significantly on the band gap of the oxide and its band edge energies relative to the potentials of relevant aqueous redox couples. Here we focus on the water interface with MgO, a prototypical wide band gap oxide whose conduction band edge is close in energy to that of water. We investigate the behavior of an excess electron at and out of equilibrium near the interface using ab initio molecular dynamics based on hybrid density functional theory. Our simulations show that under equilibrium conditions the excess electron (donated by an Al impurity in MgO) localizes to a midgap defect state comparable in energy and shape to a hydrated electron in bulk water. To characterize the electron transfer from the conduction band of MgO to interfacial product states, we dope near-equilibrium configurations of the pristine MgO-water system with Al and run short trajectories of these instantaneously out-of-equilibrium systems. We observe two distinct products associated with the excess electron: a surface-localized electron (esurf-) and an aqueous hydrogen radical (H•). The H• pathway exhibits a much higher activation barrier despite being more exoergic, making esurf- the kinetic product. Our characterization of the pathways on the basis of Marcus theory is consistent with the poor observed utility of MgO for water radiolysis. Moreover, we anticipate that the computational framework employed here will be broadly applicable to assessing electron transfer mechanisms at aqueous, photocatalytic interfaces.

Coarse-grained tight-binding models

Calculating the electronic structure of systems involving very different length scales presents a challenge. Empirical atomistic descriptions such as pseudopotentials or tight-binding models allow one to calculate the effects of atomic placements, but the computational burden increases rapidly with the size of the system, limiting the ability to treat weakly bound extended electronic states. Here we propose a new method to connect atomistic and quasi-continuous models, thus speeding up tight-binding calculations for large systems. We divide a structure into blocks consisting of several unit cells which we diagonalize individually. We then construct a tight-binding Hamiltonian for the full structure using a truncated basis for the blocks, ignoring states having large energy eigenvalues and retaining states with energies close to the band edge energies. A numerical test using a GaAs/AlAs quantum well shows the computation time can be decreased to less than 5% of the full calculation with errors of less than 1%. We give data for the trade-offs between computing time and loss of accuracy. We also tested calculations of the density of states for a GaAs/AlAs quantum well and find a ten times speedup without much loss in accuracy.

Morphology controllable synthesis of GdOF nanocrystals and application in theranostic purpose

In this present paper, GdOF nanostructures have been synthesized by the urea based homogeneous coprecipitation method combining with high temperature (500 °C) calcination in presence of N2. The nanostructures were structurally characterized by X-ray diffraction in conjunction with Rietveld refinements. The effects of pH of the reaction medium on the shape, size and morphology were explored and discussed in detail. The formation of GdOF nanostructures were represented with proper reaction mechanism elaborately. Vibrational energies of different bonds and defects states in our desired crystal structures were investigated using FTIR and Raman spectroscopy respectively. Under ultraviolet excitation, the GdOF nanomaterials show violet, blue and green emissions and get information about band edge energies. Based on its photoluminescence property, generation of reactive oxygen species (ROS) was investigated in detail.

Electronic structure of strain-tunable Janus WSSe-ZnO heterostructures from first-principles.

The electronic structure of semiconducting 2D materials such as monolayer transition metal dichalcogenides (TMDs) are known to be tunable via environment and external fields, and van der Waals (vdW) heterostructures consisting of stacks of distinct types of 2D materials offer the possibility to further tune and optimize the electronic properties of 2D materials. In this work, we use density functional theory (DFT) calculations to calculate the structure and electronic properties of a vdW heterostructure of Janus monolayer WSSe with monolayer ZnO, both of which possess out of plane dipole moments. The effects of alignment, biaxial and uniaxial strain, orientation, and electric field on dipole moments and band edge energies of this heterostructure are calculated and examined. We find that the out of plane dipole moment of the ZnO monolayer is highly sensitive to strain, leading to the broad tunability of the heterostructure band edge energies over a range of experimentally-relevant strains. The use of strain-tunable 2D materials to control band offsets and alignment is a general strategy applicable to other vdW heterostructures, one that may be advantageous in the context of clean energy applications, including photocatalytic applications, and beyond.

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.

High hole mobility in boron delta-doped layers in diamond: why it is not achieved as yet and how it can be achieved

The required parameters of nanometer boron delta-doped layers in diamond for achieving high conductivity and hole mobility are calculated. The boron concentration in such layers has to be sufficient to achieve the insulator--metal phase transition, i. e. metallic conductivity. Then, it is demonstrated that taking into account valence band edge energy shift due to the presence of ionized boron atoms leads to the significant deepening of the potential well formed by the delta-doped layer for holes. It results in much stronger hole confinement than it was expected before. Thus, it is predicted that a significant delocalization-induced increase of hole mobility can be achieved if metallic boron delta-doped layer thickness is of order and smaller than 0.5 nm and compensation ratio does not exceed 42%. Keywords: delta-doped layers, nanostructures, diamond films chemically deposited from the vapor phase, hole mobility, insulator-metal phase transition.

A Dual-Beam Wideband Filtering Patch Antenna With Absorptive Band-Edge Radiation Nulls

A dual-beam filtering patch antenna with two absorptive band-edge radiation nulls and four reflective stopband radiation nulls is proposed. By loading a pair of T-shaped metallic strip underneath the radiating patch, a modified TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sub> mode with TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> -like radiation pattern is obtained, which combines with the TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> mode of the radiating patch, generating a wide operating band with symmetrical dual-beam radiation pattern. Two reflective radiation nulls are generated in both upper and lower stopbands for suppressing the out-of-band radiation. Through embedding two pairs of shorted small patch in the radiating patch, one absorptive radiation null combining with one absorptive resonant mode is produced at each band edge to absorb the band-edge incident energy, leading to a reduced reflection and sharp roll-off at both band edges, and therefore, the transceiver can be protected. With a total height of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.038~\lambda _{0}$ </tex-math></inline-formula> , a prototype is designed, fabricated, and measured, showing a −10 dB impedance bandwidth from 4.63 to 6.10 GHz with sharp band-edge roll-off, two radiation beams directed at ±40° with an in-band gain of 6.0 dBi, and high out-of-band suppression of over 15 dB.

Effects of Zn impurity on the photoluminescence properties of InP quantum dots

Heavy-metal-free indium phosphide (InP)-based quantum dots (QDs) have attracted much attention as materials for display applications. Methods such as introducing Zn in the core of InP QDs and passivating wide band-gap materials have been used to improve the luminescent properties of InP QDs. Herein, we investigated the photophysical properties of the In(Zn)P core QDs with the influence of the Zn additive on the InP QDs. The near band edge photoluminescence (PL) of the In(Zn)P QDs was spectrally asymmetric in shape, with a redshifted side peak originating from a Zn impurity that induced shallow defect sites, resulting in PL linewidth broadening. The shallow trap emission of the In(Zn)P QDs disappeared after the overcoating of the ZnSeS passivated the shell layers, and the PL linewidth of In(Zn)P/ZnSeS (core/shell) QDs was solely determined by the band-edge excitonic energy distribution caused by the size dispersion in ensemble QD systems. We expect that the understanding the photophysical properties of In(Zn)P-based QDs future strategies to narrow the ensemble emission line of the InP-based QD synthesis.

On topological materials as photocatalysts for water splitting by visible light

We performed a virtual materials screening to identify promising topological materials for photocatalytic water splitting under visible light irradiation. Topological compounds were screened based on band gap, band edge energy, and thermodynamics stability criteria. In addition, topological types for our final candidates were computed based on electronic structures calculated usingthe hybrid density functional theory including exact Hartree–Fock exchange. Our final list contains materials which have band gaps between 1.0 and 2.7 eV in addition to band edge energies suitable for water oxidation and reduction. However, the topological types of these compounds calculated with the hybrid functional differ from those reported previously. To that end, we discuss the importance of computational methods for the calculation of atomic and electronic structures in materials screening processes.

Strain-modified effective two-band model for calculating the conduction band structure of strain-compensated quantum cascade lasers: effect of strain and remote band on the electron effective mass and nonparabolicity parameter

We propose a strain-modified effective two-band model to calculate the conduction band (CB) structure of strain-compensated quantum cascade lasers (QCLs). The proposed model can consider the effect of strain and remote band (RB) on the band-edge energy, electron effective mass, and nonparabolicity parameter although the currently used empirical two-band model can be applicable to only the unstrained QCLs. Based on the three-band second-order k·p Hamiltonian along with the Pikus-Bir Hamiltonian, analytical formula for the electron effective mass and nonparabolicity parameter are derived at the zone center, where the effects of strain and RB interaction are included. Then, the three-band first-order k·p Hamiltonian is reduced to the strain-modified effective two-band Hamiltonian, where the effective Kane energy, determined by the electron effective mass and nonparabolicity parameter, is used to include the nonparabolicity of the CB. By numerically solving the proposed strain-modified effective two-band model based on the finite difference method, we calculate the CB structure of several strain-compensated or unstrained QCLs in the mid-IR and terahertz range and predict their lasing wavelengths, which are well matched with the measured values in the literatures.