Center Of Band Gap Research Articles

Effect of defects on high efficient perovskite solar cells

Abstract The organic-inorganic hybrid halide perovskite solar cell has been studied theoretically in this paper. For the validation cases, an experimental case is used for comparison with our simulation results, and good agreement was achieved between them. Here, the effect of defect properties such as defect density, defect position, and capture cross-section for electron and hole in the perovskite layer were studied. The results show that the deep defects (near to the center of perovskite band gap) are identified as harmful defects due to the high Shockley-Read-Hall recombination rate. In high defect density, the power conversion efficiency (PCE) dramatically reduced. The same behavior is observed for the electrons-holes capture cross-section. Finally, we classified harmful and benign defects in perovskite solar cells.

Structural stability, surface and photocatalytic properties of CdGeP2: A DFT study

The aim of this study was to investigate the photocatalytic properties of tetragonal cadmium germanium diphosphide. In order to achieve this, stability probe and a careful examination of the surface properties is crucial. From our analysis done using the first principle technique, this ternary compound displays structural, mechanical and vibrational stability based on its cohesive energy, elastic constants and phonon dispersion. Moreover, from the values of the computed band edge positions that were obtained using both the Mulliken and band gap center techniques, the ternary CdGeP2 is able to straddle the water redox potential. Thus, it shows the viability to split water into hydrogen and oxygen, resulting in clean energy generation. Hence, this study provides a worthwhile undertaking towards solving the impending energy crisis through the identification, characterization and optimization of materials that are exceptional for artificial photocatalytic applications.

Two-dimensional phononic crystal sensor for volumetric detection of hydrogen peroxide (H2O2) in liquids

In this work, a novel design of a highly sensitive two-dimensional fluid/fluid phononic crystal sensor for sensing hydrogen peroxide (H 2 O 2 ) concentrations up to 75.5% in water is presented. The transmission spectra of the phononic crystal sensor is calculated based on the finite element method (FEM). The host matrix is made from mercury and the inclusions are filled with water. H 2 O 2 with different concentrations have been added in a cavity inside the phononic crystal design. Different resonant modes related to each H 2 O 2 concentration were localized in the center of the phononic band gap. It is predicted that the central frequency of this resonant mode changes as the concentration of the H 2 O 2 alters. The sensor showed high performance and quality factor versus a wide range of H 2 O 2 concentrations. The sensitivity reached up the value of 426 Hz for each 1% change in H 2 O 2 concentration. Also, we obtained a quality factor with the value of 1612 and a figure of merit with the value of 2488. Thereby this sensor can be used as a highly sensitive biosensor for hydrogenates, acidity and all components such as glucose and uric acid in the blood. Finally, obtained simulation results revealed that the proposed H 2 O 2 sensor can be considered as a competitive, prospective, flexible and commercial device for detecting and distinguishing H 2 O 2 concentrations with high efficiency. • A highly sensitive two-dimensional phononic crystal sensor is proposed to distinguish hydrogen peroxide -water mixtures. • The sensor structure from mercury as a host matrix and water inclusions. • The proposed sensor showed high performance and quality factor versus a wide range of hydrogen peroxide concentrations. • The sensor has a sensitivity of the value 426 Hz for each 1% change in H 2 O 2 concentration.

Closed-form formulas for bandgap estimation and design of metastructures undergoing longitudinal or torsional vibration

Metastructures can be employed to control waves/vibrations owing to their unique bandgap behaviour. There are two problems that are of great concern in the design of metastructures: (1) How to predict the bandgap frequency range quickly and accurately? (2) How to design a desired bandgap with broad width and large attenuation performance within the weight and size constraints? This work aims to explore these problems by addressing a kind of metastructures composed of a periodic array of local resonators attached to a host structure undergoing longitudinal/torsional vibration. Such structures have both fundamental and practical significance. For the first problem, closed-form formulas of the bandgap edge and centre frequencies are derived. These formulas enable a direct and accurate estimation of the lowest two bandgaps of the considered metastructures within the entire design space, being more general and applicable than well-known existing closed-form formulas, which are only applicable to the lowest bandgap created in the deep-subwavelength regime. For the second problem, a guideline is proposed for the design of a sub-wavelength region bandgap with simultaneously significant and robust attenuation performance at its centre frequency, and an improved bandgap width than the conventional deep-subwavelength design. Furthermore, closed-from design formulas for optimal selection of system parameters are provided. The applicability of the proposed closed-form formulas and guideline is validated by numerical examinations of simple and advanced realizable metastructures.

A machine learning based approach for phononic crystal property discovery

Phononic crystals are artificially structured materials that can possess special vibrational properties that enable advanced manipulations of sound and heat transport. These special properties originate from the formation of a bandgap that prevents the excitation of entire frequency ranges in the phononic band diagram. Unfortunately, identifying phononic crystals with useful bandgaps is a problematic process because not all phononic crystals have bandgaps. Predicting if a phononic crystal structure has a bandgap, and if so, the gap's center frequency and width is a computationally expensive process. Herein, we explore machine learning as a rapid screening tool for expedited discovery of phononic bandgap presence, center frequency, and width. We test three different machine learning algorithms (logistic/linear regression, artificial neural network, and random forests) and show that random forests performs the best. For example, we show that a random phononic crystal selection has only a 17% probability of having a bandgap, whereas after incorporating rapid screening with the random forests model, this probability increases to 89%. When predicting the bandgap center frequency and width, this model achieves coefficient of determinations of 0.66 and 0.85, respectively. If the model has a priori knowledge that a bandgap exists, the coefficients of determination for center and width improve to 0.97 and 0.85, respectively. We show that most of the model's performance gains are achieved for training datasets as small as ∼5000 samples. Training the model with just 500 samples led to reduced performance but still yielded algorithms with predictive values.

Highly Localized Defect Mode in Polymer-Based THz Photonic Crystals Fabricated Using Stereolithography

A polymer-based one-dimensional photonic crystal with a defect mode was demonstrated for the terahertz frequency range. The photonic crystal was designed to achieve a photonic bandgap centered around 100 GHz with a narrow defect mode at the center frequency. The defect mode allowed a narrow band transmission at the center of photonic bandgap, while the transmitted signal was suppressed in the rest of the bandgap. The photonic crystal incorporated two identical sets of alternating compact and low-density layer pairs symmetrically enclosing a defect layer. The compact layers consisted entirely of polymethacrylate with no intentional internal structures, while the low-density layers were composed of sub-wavelength-sized columns. The columnar structures had a volumetric fraction selected to provide a desired index contrast between the adjacent layers. The photonic crystal samples were fabricated in a single-step stereolithography using a commercial system. THz transmission spectroscopy measurements were carried out to determine the optical response of the sample in a range from 82 to 125 GHz. Stratified optical layer model calculations were used to evaluate the transmission data. A distinct photonic bandgap with a defect mode centered at 99 GHz was observed in the experimental transmission spectra. A good agreement between the relevant model parameters and the corresponding design parameters was found.

Bandgap-tunable device realized by ternary plasma photonic crystals arrays

We reported a bandgap-tunable device with ternary plasma photonic crystals (PPCs), achieving tunable bandgap for controlling the propagation of free-space electromagnetic waves from 11.5 GHz to 14.5 GHz. The device is designed as a square crystal structure composing ternary PPCs arrays. Both simulation and experimental results indicate that the transmission of the electromagnetic waves can be controlled by changing the plasma frequency, dielectric constant, and structure spacing in the device, realizing the dynamic adjustment of photonic bandgap bandwidth and center frequency. In addition, the plasma frequency was measured, which is consistent with the simulation results. Our strategy can be applied to design a variety of devices, including reconfigurable antennas, plasma lenses, and military-developed stealth equipment.

Electric-Field-Regulated Energy Transfer in Chiral Liquid Crystals for Enhancing Upconverted Circularly Polarized Luminescence through Steering the Photonic Bandgap.

Circularly polarized luminescent materials with high dissymmetry factor (glum ) have been attracting increasing attention due to their distinctive photonic properties. In this work, by incorporating upconversion nanoparticles (UCNPs) and CsPbBr3 perovskite nanocrystals (PKNCs) into a chiral nematic liquid crystal (N*LC), enhanced upconverted circularly polarized luminescence (UC-CPL) based on a radiative energy transfer (RET) process from UCNPs to CsPbBr3 PKNCs is successfully implemented. By locating the emission peak of CsPbBr3 PKNCs at the center of the photonic bandgap of N*LC, the maximum glum value of UC-CPL can be amplified to an extremely large value of 1.1. Meanwhile, upconverted emission of UCNPs can be significantly enhanced due to the band edge enhancement effect of the N*LC, subsequently enhancing the emission of the CsPbBr3 PKNCs through the RET process. In addition, an applied electric field can switch the upconverted emission of the UCNPs, as well as the RET process, enabling an electric-field-controlled UC-CPL switch.

Multiple parameter space bandgap control of reconfigurable atmospheric plasma photonic crystal

A plane wave expansion method is used to simulate the bandgaps for a square lattice plasma photonic crystal over a parameter space of five independent variables, characteristic of a reconfigurable atmospheric discharge (plasma frequency: 0.056–5.6 × 1012 rad/s, collision frequency: 1–6 × 1012 rad/s, plasma column radius: 0.2–0.4 mm, lattice constant: 1–2 mm, and background dielectric: 1–10). The trends of the first and second bandgaps allow for five-dimensional 4th order polynomial equations to be fitted to the data, defining the boundaries of the first and second bandgaps with 8% and 2% error, respectively. Performance metrics (operational frequency range and frequency sensitivity) of the plasma photonic crystal are defined and evaluated for each controlling variable. The results show that, within the variable space investigated here, the column radius and background dielectric are the most effective controlling variables for the bandgap bandwidth and center frequency, respectively. The maximum frequency range provided by the variable ranges investigated here is 25–400 GHz and 0–250 GHz for the TE1 and TE0 bandgaps, respectively.

Cr-Triggered Local Structural Change in Cr2Ge2Te6 Phase Change Material.

Cr2Ge2Te6 (CrGT) is a phase change material with higher resistivity in the crystalline phase than in the amorphous phase. CrGT exhibits an ultralow operation energy for amorphization. In this study, the origin of the increased resistance in crystalline CrGT compared to amorphous CrGT and the underlying phase change mechanism were investigated in terms of both local structural change and associated change in electronic state. The density of states at the Fermi level in crystalline CrGT decreased with increasing annealing temperature and became negligible upon annealing at 380 °C. Simultaneously, the Fermi level shifted from the vicinity of the valence band to the band gap center, leading to an increase in resistance. The phase change from amorphous to crystalline CrGT occurred through a metastable crystalline phase with a local structure similar to that of the amorphous phase. Cr nanoclusters were confirmed to exist in both the amorphous and crystalline phases. The presence of Cr nanoclusters induced Cr vacancies in the crystalline phase. These Cr vacancies generated hole carriers, leading to p-type conduction. Photoelectron spectroscopy of the Cr 2s core level clearly indicated a decrease in the fraction of Cr-Cr bonds and an increase in the fraction of Cr-Te bonds in crystalline CrGT upon annealing. Meanwhile, the coordination number of the Cr nanoclusters decreased as the number of Cr-Cr bonds was reduced. Together, these results imply that the origin of the increased resistance in crystalline CrGT is the filling of Cr vacancies by Cr atoms diffusing from Cr nanoclusters.

Locally Resonant Phononic Crystals at Low frequencies Based on Porous SiC Multilayer

In this work, a one-dimensional porous silicon carbide phononic crystal (1D-PSiC PnC) sandwiched between two rubber layers is introduced to obtain low frequency band gaps for the audible frequencies. The novelty of the proposed multilayer 1D-PnCs arises from the coupling between the soft rubber, unique mechanical properties of porous SiC materials and the local resonance phenomenon. The proposed structure could be considered as a 1D acoustic Metamaterial with a size smaller than the relevant 1D-PnC structures for the same frequencies. To the best of our knowledge, it is the first time to use PSiC materials in a 1D PnC structure for the problem of low frequency phononic band gaps. Also, the porosities and thicknesses of the PSiC layers were chosen to obtain the fundamental band gaps within the bandwidth of the acoustic transducers and sound suppression devices. The transmission spectrum of acoustic waves is calculated by using the transfer matrix method (TMM). The results revealed that surprising low band gaps appeared in the transmission spectra of the 1D-PSiC PnC at the audible range, which are lower than the expected ones by Bragg’s scattering theory. The frequency at the center of the first band gap was at the value 7957 Hz, which is 118 times smaller than the relevant frequency of other 1D structures with the same thickness. A comparison between the phononic band gaps of binary and ternary 1D-PSiC PnC structures sandwiched between two rubber layers at the micro-scale was performed and discussed. Also, the band gap frequency is controlled by varying the layers porosity, number and the thickness of each layer. The simulated results are promising in many applications such as low frequency band gaps, sound suppression devices, switches and filters.

Triple Notches Bandstop Microstrip Filter Based on Archimedean Spiral Electromagnetic Bandgap Structure

With the development of artificial electromagnetic structures, defective grounding structures (DGS), defective microstrip structures (DMS), and electromagnetic bandgap (EBG) have been widely used in the design of microstrip filters. In this paper, a triple notches ultra wideband bandstop microstrip filter based on Archimedean spiral electromagnetic bandgap structure (ASEBG) structure is proposed. Firstly, the equivalent circuit of ASEBG is analyzed, and L and C values are extracted by using Advanced Design System (ADS). Secondly, the correctness of the lumped parameter model is verified by comparing the High Frequency Structure Simulator (HFSS) simulation results with the measured results. Finally, the influence of ASEBG structure parameters on resonant performance is analyzed by HFSS simulation, and the filter parameters are further optimized. By coupling ASEBG structure to existing double notch microstrip filters, a triple notches ultra wideband bandstop microstrip filter is realized. This method can also be used in the design of other microstrip devices with stopband characteristics. The three bandgap center frequencies of the proposed triple notches ultra wideband bandstop microstrip filter are 3.5, 5.2, and 7.4 GHz, respectively. The corresponding maximum attenuation of the three stopbands is 33.6, 24.8, and 21.7 dB, respectively.

Bloch band gap of shallow-water waves over infinite arrays of parabolic bars and rectified cosinoidal bars and Bragg resonance over finite arrays of bars

In this paper, eigenvalue problems of Bloch waves over an infinite array of equally spaced parabolic bars and rectified cosinoidal bars in shallow water are respectively studied. Both band expressions in closed form for the two eigenvalue problems are derived, which depend upon the dimensionless bar height with respect to water depth and the dimensionless bar width and spacing with respect to incident wavelength. It is found that the band-gap center associated with an infinite array of bars coincides well with the peak phase of Bragg resonance by any finite array of bars. For any fixed dimensionless bar height, a gap map as a bivariate function of the dimensionless bar width and spacing can be given. When both the dimensionless bar height and width are fixed, the band gap occurs periodically. When the dimensionless bar height is fixed, there exists a particular dimensionless bar width such that the band gap achieves its maximal width. Based on the gap maps, the well-known phenomenon of phase downshift can be visually and quantitatively explained, and the peak phase of Bragg resonant reflection by a finite array of bars can be well predicted, which is much more accurate than that predicted by Bragg’s law.

Improving Electrochemical Pb2+ Detection Using a Vertically Aligned 2D MoS2 Nanofilm.

Recent advancements in MoS2 nanofilms have aided in the development for important water-related environmental applications. However, a MoS2 nanofilm-coated sensor has yet to have been applied for heavy metal detection in water-related environmental samples. In this study, a novel vertically aligned two-dimensional (2D) MoS2 (edge exposed) nanofilm was applied for in situ lead ion (Pb2+) detection. The developed sensor showed an excellent linear relationship toward Pb2+ between 0 and 20 ppb at -0.45 V vs Ag/AgCl using square wave anodic stripping voltammetry (SWASV) with the improved limit of detection (LOD) of 0.3 ppb in a tap water environment. The vertically aligned 2D MoS2 sensor exhibited improved detection sensitivity (2.8 folds greater than a previous metallic [Bi] composite electrode) with lower relative standard deviation for repetitive measurements (n = 11), indicating enhanced reproducibility for Pb2+ detection. The vertically aligned 2D MoS2 layers exhibited 2.6 times higher sensitivity than horizontally aligned 2D MoS2 (basal plane exposed). Density functional theory calculations demonstrated that adsorption energy of Pb on the MoS2 side edge was much higher (4.11 eV) than those on the basal plane (0.36 and 0.07 eV). In addition, the band gap center of vertical MoS2 was found to be higher than the Pb2+ → Pb reduction potential level and capable of reducing Pb2+. Overall, the newly developed vertically aligned 2D MoS2 sensor showed excellent performance for detecting Pb2+ in a real drinking water environment with good reliability.

Rapid fabrication and characterization of SiO2-Fe2O3 heterogeneous metamaterial via one-step self-assembly process

SiO2-Fe2O3 heterogeneous metamaterial was fabricated by one-step self-assembly process. The micro-morphology and optical properties of the SiO2-Fe2O3 heterogeneous metamaterials were characterized by scanning electron microscope (SEM) and UV/Vis/NIR spectrophotometer respectively. The crystal phase of the calcined Fe2O3 powders was characterized by X-ray diffraction (XRD). The influences of the concentration of SiO2 suspension and voltage on heterogeneous metamaterials are discussed. The model of SiO2-Fe2O3 heterogeneous photonic crystal was established, and the wavelength of the bandgap center was simulated and analyzed. The results show that the ordered SiO2-Fe2O3 heterogeneous metamaterial can be successfully obtained by a one-step self-assembly process with no voltage when the concentration of SiO2 suspension is 3% (in a short time). In the suspension system, the voltage is not beneficial to the self-assembly process, and even destroys the electrode cathode. As can be seen from transmission spectra, the as-prepared samples have three bandgaps, and the absorption peaks are obvious in the near infrared range (1235˜1419 nm). In this paper, we show that the application of this composite system can avoid dependence on the ITO electrode material.

Electrical transport mechanism of the amorphous phase in Cr2Ge2Te6 phase change material

A Cr2Ge2Te6 (CrGT) phase change material (PCM) was studied. Different from conventional PCMs, it shows an inverse resistance change between a low-resistance amorphous phase and a high-resistance crystalline phase. Moreover, the anomalous low resistivity in the amorphous CrGT is considered to be due to a large carrier density, but the mechanism of electrical transport is still not clear. In this study, the electrical transport mechanism of the amorphous CrGT was discussed based on the temperature dependence of the resistivity, carrier density, mobility, and current–voltage characteristics. Above 300 K, the conduction mechanism of the amorphous CrGT was thermally activated band conduction, which is different from the conventional Ge–Sb–Te PCMs that show Poole–Frenkel conduction in the amorphous phase. Below 300 K, the amorphous CrGT shows hopping conduction, changing from variable range hopping (Mott VRH) to Efros–Shklovskii variable range hopping (ES-VRH) with decreasing temperature. The crossover from Mott VRH to ES-VRH was observed at around 200 K. Furthermore, the Fermi level was not pinned at the center of bandgap; instead, it was located near the valence band.

Exploring half-metallic Co-based full Heusler alloys using a DFT+U method combined with linear response approach.

A density functional theory (DFT)+U method based on linear response (LR) theory was applied to investigate the electronic structures of a Co-based ternary full Heusler alloy Co2YSi to explore half-metallic (HM) ferromagnets with a wide HM gap. The LR-based DFT+U calculations tend to obtain a reasonable correlation parameter for the Y site, while the correlation of the Co site misdirects to the unphysical ground state due to the overestimated parameter value that arises from the delocalized electronic structure of Co. Furthermore, we found that the HM gap of Co2MnSi originates from the Co orbital in the conduction state and the Co–Mn hybridizing t2g orbital in the valence state around the Fermi energy. This means that the HM gap is a tunable property by selecting the Y element and/or mixing several elements into the Y site through t2g atomic-orbital coupling. Our LR-based DFT+U method was extended to other ternary Co2YSi and quaternary Co2(Y,Mn)Si. We found that Co2(Ti0.25,Mn0.75)Si and Co2(Fe0.25,Mn0.75)Si show HM nature, with the Fermi energy being at almost the center of the minority band gap, which leads to high thermal stability.

Deep level transient spectroscopic analysis of p/n junction implanted with boron in n-type silicon substrate

For P-i-N diodes implanted and activated with boron ions into a highly-resistive n-type Si substrate, it is found that there is a large difference in the leakage current between relatively low temperature furnace annealing (FA) and high temperature laser annealing (LA) for activation of the p-layer. Since electron trap levels in the n-type Si substrate is supposed to be affected, we report on Deep Level Transient Spectroscopy (DLTS) measurement results investigating what kinds of trap levels are formed. As a result, three kinds of electron trap levels are confirmed in the region of 1–4 μm from the p-n junction. Each DLTS peak intensity of the LA sample is smaller than that of the FA sample. In particular, with respect to the trap level which is the closest to the silicon band gap center most affecting the reverse leakage current, it was not detected in LA. It is considered that the electron trap levels are decreased due to the thermal energy of LA. On the other hand, four kinds of trap levels are confirmed in the region of 38–44 μm from the p-n junction and the DLTS peak intensities of FA and LA are almost the same, considering that the thermal energy of LA has not reached this area. The large difference between the reverse leakage current of FA and LA is considered to be affected by the deep trap level estimated to be the interstitial boron.

Optimization of one-dimensional photonic crystals with double layer magneto-active defect

Success of practical implementation of one-dimensional photonic crystals with magneto-active layers is evaluated in high values of magneto-optical (MO) quality factor Q and figure of merit F. The article relates to optimization of one-dimensional photonic crystals with double layer magneto-active (MA) defect of composition Bi1.0Y0.5Gd1.5Fe4.2Al0.8O12/Bi2.8Y0.2Fe5O12 located between the nongarnet dielectric Bragg mirrors. The structure design was performed by changing the number of layer pairs in Bragg mirrors m and the optical thickness of MA defect lM to achieve high values of MO characteristics. Theoretical predictions were confirmed by experimental investigation of eight synthesized configurations with m = 4 and m = 7. We have demonstrated the maximum Q = 15.1 deg and F = 7.5% at 624 nm for structure with m = 4 and lM = (2.5·λ0/2), where λ0 = 690 nm is the photonic band gap center. Configurations with m = 3 can also provide their effectiveness in realization. Maximum MO activity was achieved for configurations with m = 7. The structures with lM = (0.8·λ0/2) and lM = (2.5·λ0/2) showed respectively the specific Faraday rotation −113 deg/μm (that exceeds in 62 times the Faraday rotation of MA double layer film) at 654 nm and absolute Faraday rotation −20.6 deg at 626 nm.

Band alignments of graphene-like III-nitride semiconductors

Band alignment in graphene-like III-nitride semiconductors was investigated using first principles calculations and an empirical formula. We estimated the band edge positions using two simple approaches based on the energy of the band gap center (BGC) and electron affinity rules. The energy positions relative to the vacuum level were determined from the BGC and Mulliken electronegativities. These methods provided similar trends in the band lineup. The valence band and conduction band offsets determined by the first principles calculations agreed with the empirical results to within 0.5 eV. The findings suggest that the first principles and empirical methods provide a useful guide for high-throughput device design.