Band Structure Of Materials Research Articles

Tunable Optical Activity in Twisted Anisotropic Two-Dimensional Materials.

Twisted van der Waals structures exhibit a variety of unusual electrical and optical phenomena and could provide a powerful means for designing nanodevices with tunable chiral properties. However, programming intrinsic chiral properties of the film on the atomic scale remains a great challenge due to the limitations of fabrication and measurement techniques. Here, we report a highly tunable large optical activity of twisted anisotropic two-dimensional (2D) materials, including black phosphorus (BP), ReS2, PdSe2, and α-MoO3, by varying the twist angle between the stacked layers. The chirality can be deliberately tailored through the engineering of the symmetry, band structure, and anisotropy of 2D materials, demonstrating the high tunability of the chirality. The results show the highest thickness-normalized ellipticity value (13.8 deg μm-1, twisted ReS2) and ellipticity value (1581 mdeg, twisted BP) among the systems based on 2D materials. It is also shown that the chiroptical response exists in an extremely large spectral range from the visible to the infrared. Furthermore, the twisted ReS2 enabled spin-selective control of the information transformation. These results show that highly controllable chirality in twisted 2D anisotropic materials has considerable potential in on-chip polarization optics, nano-optoelectronics, and biology.

High‐Performance Te‐Doped PbSe Film Heterojunction Photodetector with Current Rectification Effect and Broadband Detection Capability

AbstractPbSe has attracted wide attention owing to its fascinating physical and chemical properties. It has important technical significance for infrared detectors. However, due to the inherent bandgap, low carrier mobility, and dielectric constant of PbSe, it is a significant challenge to realize fast response and mid‐infrared (MID) photodetection. Chemical doping is an efficient method to regulate the band structure and carrier mobility of materials. Nevertheless, the doped film detector cannot satisfy the demands of high‐performance photodetectors. In this paper, different composition ratios powders are synthesized by a solid‐state method. The PbSe0.5Te0.5 alloy film displays narrower bandgap, higher carrier mobility, excellent photo‐response, and broadband detection capacity. By utilizing the above superiority of PbSe0.5Te0.5 and fast charge transfer in MoSe2, a PbSe0.5Te0.5/MoSe2 p‐n heterostructure device is successfully fabricated that has significant rectifying effect (103) and extremely low dark current density (720 µA cm−2). The photodetector has a maximum responsivity (R) and specific detectivity (D*) of 17.5 A W−1 and 3.08 × 1013 Jones at 780 nm. In addition, the detector displays superior photodetection performance with broadband photodetection (405–5000 nm), high switch ratios (Ion/Ioff = 105), and fast response speed. The above outstanding properties indicate that the PbSe0.5Te0.5/MoSe2 heterostructure provides a promising and efficient strategy for achieving a miniaturized, broadband, and high‐performance photodetector.

Toward first-principles approaches for mechanistic study of self-trapped exciton luminescence

In recent years, broadband photo-luminescence phenomena arising from self-trapped exciton (STE) in metal halides, including perovskites and various low-dimensional derivatives and variants, have attracted increasing attention for their potential diverse optoelectronic applications like lighting, display, radiation detection, and sensing. Despite great success in experimental discovery of many efficient STE emitters, the current understanding of the STE emission mechanism in metal halides is still immature, and often controversial, which calls for help urgently from predictive first-principles theoretical calculation. Although density-functional theory (DFT) based calculations are routinely used to provide electronic band structure of materials and have contributed greatly to qualitative analysis of luminescence mechanism, more in-depth and quantitative information is highly needed to provide guidelines for rational design of new luminescent materials with desirable features. However, due to the complicated nature of STE emission, involving in particular electron–phonon coupling in both ground and excited states, the usage of DFT is no longer a routine job as for ground state properties. While more sophisticated methods formulated in the framework of many-body perturbation theory like GW-Bethe–Salpeter equation are available and provide theoretically rigorous and accurate description of electronic transitions in extended systems, their application to real STE systems is still severely limited due to highly demanding computational cost. In practice, approximated DFT methods are employed, which have their own strengths and limitations. In this review, we focus on the theoretical approaches that have been heavily used in interpreting STE luminescence mechanism, with a particular emphasis on theoretical methods for exciton self-trapping structural optimization. It is hoped that this review, by summarizing the current status and limitations of theoretical research in the STE emission, will motivate more methodological development efforts in this important field, and push forward the frontiers of excited state electronic structure theory of materials in general.

High-energy-resolution angle-resolved inverse-photoelectron spectroscopy apparatus for damage-free measurements of conduction band structures of functional materials.

The energy band structure of the conduction band (energy-momentum relation of electrons) is crucial to understanding the electron transport of crystalline materials. In this paper, we describe an angle-resolved low-energy inverse photoelectron spectroscopy (AR-LEIPS) apparatus that examines the conduction band structures of materials sensitive to the electron beam, such as organic semiconductors and organic-inorganic hybrid perovskites. The principle of this apparatus is based on AR inverse photoelectron spectroscopy. To minimize radiation damage and improve energy resolution, we employed our previous approach used in LEIPS [H. Yoshida, Chem. Phys. Lett. 539-540, 180 (2012)]. We obtained an overall energy resolution of 0.23eV with a momentum resolution of 0.9nm-1 at the electron kinetic energy of 2eV or higher.

Origin of improved average power factor and mechanical properties of SnTe with high-dose Bi2Te3 alloying

Chemical bond and defect engineering have profound impact on energy band and crystal structure of materials, which can adjust physicochemical and mechanical properties of materials. BiSn and VSn are simultaneously designed in SnTe material system via the (SnTe)x(Bi2Te3) alloying form, which was realized by vacuum melting and spark plasma sintering technology. Both point defects increase band gap and decrease ΔEL−Σ, which improves obviously Seebeck coefficient of SnTe. The high-efficiency improvement in the middle temperature zone makes average power factor of (SnTe)24(Bi2Te3) achieves 1656 μWm-1K−2(323K–773K). BiSn induces strong coupling hybridization between Te-5s and Te-5p, and forms the lower bonding state. The lower bonding state and the larger -IpCOHP increase bond strengthen, which result in the smaller thermal expansion coefficient and the higher hardness. Bonding evolution makes thermal expansion coefficient of SnTe reduce by 27%, and Vickers hardness increase by 44%. Higher average power factor, lower thermal expansion coefficient and higher hardness can improve output power density and service life, which provides strategies for exploiting high-power thermoelectric devices with suitable mechanical properties.

An effective strategy of indium half-doping to stabilize the structure and improve optoelectronic characteristics of Tl-Co based double perovskite: First principles study

Doping can improve the structural stability and photoelectric properties of perovskite materials. In this study, indium element was added into Tl-Co double based perovskite material to stabilize its structure and improve its photoelectric performance. The results show that the doped material Cs2In0.5Tl0.5CoX6(X = I, Br, Cl) has better ductility and stability than the original Cs2TlCoI6 by elastic constants and phonon spectra. Moreover, indium doping can improve the band structure of Tl-Co double-base perovskite materials, such as changing the band value to the optimal band gap value of solar energy absorber and transforming from indirect band structure to quasi-direct band structure. In terms of optical properties, indium doping can expand the wavelength range of the material's light absorption, making its absorption spectrum more matching with the simulated AM1.5 spectrum. Cs2In0.5Tl0.5CoCl6 is proposed to be a promising candidate for optoelectronic applications based on its structural stability, bandgap value and optical absorption spectrum.

New maxen-based heterostructures for solar energy harvesting

A systematic study of the synthesis and photoactive properties of MXene heterostructures from four-component MAX phases is proposed for the first time. Important fundamental knowledge on the energy band structure of such materials was obtained, which allowed to explain the photoexcitation and charge transfer processes in photocatalytic compositions of this class. The mechanism of catalytic action of MXene heterostructure compositions was studied. Conducted research extends the field of fundamental knowledge about MXene heterostructures and provides a basis for their practical application, including the transition to new materials for the needs of alternative energy.

A multifunctional optical‐thermal logic gate sensor array based on ferroelectric BiFeO3 thin films

AbstractThe growing need to process a diverse range of data has ignited effort in developing new multifunctional logic gate devices. In this article, we report a new form of all‐in‐one logic gate system that exploits the photoresponsivity of a self‐powered multifunctional BiFeO3 (BFO) sensor material. The BFO sensor can not only detect both light intensity and temperature, but it can also execute three common logic gates of “AND”, “OR”, and “NOT” by converting optical and thermal inputs into electrical output. The diverse functionality of the BFO logic gate sensor array utilizes the unique light‐and temperature‐controlled energy band structure and carrier behavior of the BFO material. To demonstrate the potential, a 3 × 3 logic gate sensor matrix is developed, which successfully detected light and temperature distributions, and accurately produced the three basic logic gate operations. This work provides a new route to construct highly integrated multifunctional electronic devices for the advancement of large sensing, communication, and computing operations.image

Large Negative Magnetoresistance in All‐2D‐Materials‐Based Spin Valves

Two‐dimensional (2D) magnetic materials have attracted enormous interest and opened up a new direction to novel spintronic applications. Herein, a large negative magnetoresistance (MR) in all‐2D‐materials‐based vertical magnetic spin valves with nano/micro bubbles existing at the interface between the spacer layer (MoS2) and the top ferromagnetic electrode layer (Cr0.85Te or Fe3GeTe2) is demonstrated. The large negative MR, with a magnitude of ≈21.5% attained in Cr0.85Te/MoS2/Fe3GeTe2, could be fully suppressed by the elimination of the bubbles after a short argon‐protected annealing, indicating the determining role of the bubbles in modulating the spin‐related transport behavior. First‐principles calculations based on density function theory (DFT) illustrate that the compressive strain can significantly modify the spin‐resolved electronic band structures of the 2D ferromagnetic materials, and hence alter the spin polarization near the Fermi level, accounting for the negative MR phenomena with the conventional two‐current model. Herein, a new route to all‐2D‐materials‐based spintronic devices via strain engineering is provided, and also offers a new insight into the role of bubbles formed during fabricating 2D heterostructures in developing exotic properties and novel functionalities.

The Effect of the Pre-Strain Process on the Strain Engineering of Two-Dimensional Materials and Their van der Waals Heterostructures

Two-dimensional (2D) materials and their van der Waals stacked heterostructures (vdWH) are becoming the rising and glowing candidates in the emerging flexible nanoelectronics and optoelectronic industry. Strain engineering proves to be an efficient way to modulate the band structure of 2D materials and their vdWH, which will broaden understanding and practical applications of the material. Therefore, how to apply desired strain to 2D materials and their vdWH is of great importance to get the intrinsic understanding of 2D materials and their vdWH with strain modulation. Here, systematic and comparative studies of strain engineering on monolayer WSe2 and graphene/WSe2 heterostructure are studied by photoluminescence (PL) measurements under uniaxial tensile strain. It is found that contacts between graphene and WSe2 interface are improved, and the residual strain is relieved through the pre-strain process, which thus results in the comparable shift rate of the neutral exciton (A) and trion (AT) of monolayer WSe2 and graphene/WSe2 heterostructure under the subsequent strain release process. Furthermore, the PL quenching occurred when the strain is restored to the original position also indicates the pre-strain process to 2D materials, and their vdWH is important and necessary for improving the interface contacts and reducing the residual strain. Thus, the intrinsic response of the 2D material and their vdWH under strain can be obtained after the pre-strain treatment. These findings provide a quick, fast and efficient way to apply desired strain and also have important significance in guiding the use of 2D materials and their vdWH in the field of flexible and wearable devices.

Construction of Ni-contained zeolitic imidazolate framework / carbon nitride composite with modified photoelectric properties for enhanced photocatalytic degradation

ZIF/CN and Ni-contained ZIF/CN composite catalysts were successfully synthesized via two-step strategy. The physicochemical properties characterized by various methods indicated that the two composite catalysts possessed a higher surface area than as-prepared CN. Ni-ZIF/CN catalyst showed the beneficial effects on visible light absorption, electron-hole carrier generation, transfer and separation. The photocatalytic properties of the catalysts were examined via methyl orange (MO) degradation. Both of ZIF/CN and Ni-ZIF/CN exhibited the high decolorization rates above 99%, and the high reaction rate constant which is twice than that of CN. It can be concluded that ZIF-modified carbon nitride photocatalysts possess excellent photoelectrochemical properties and photocatalytic activity, giving a new pathway for modifying the band structures of materials to enhanced photocatalytic properties.

Enormous enhancement of thermoelectric properties via piezo-gating effect

The thermoelectric effect is one of the most promising techniques to generate electricity from waste heat energy. The commonly adopted approaches to enhance the thermoelectric performance limit the power factor because of the contradictory behavior between the Seebeck coefficient and electrical conductivity. This hurdle can be overcome via piezo-gating effect, which can modify the electronic band structure of active materials through strain. In this study, we developed piezo-gated flexible transistor (PGFT) comprised of ZnO thin-film (TFPGFT) and ZnO nanowires (NWPGFT) on polyethylene terephthalate (PET) as a flexible substrate and demonstrate the simultaneous enhancement of the Seebeck coefficient and conductivity by incorporating piezo-gating effect. The TFPGFT shows a gauge factor of ∼115, much higher than NWPGFTs. Further, the TFPGFT shows a higher Seebeck coefficient enhancement ∼12 times (∼0.13 to ∼1.76 µV/K) under strain resulting from the dominating piezo-gating effect. Therefore, the synergistic piezo-gating and thermoelectric effect dramatically boosts the power factor of TFPGFT by approximately 400 times (from ∼0.36 mW/K2 to ∼140.3 mW/K2), which can improve the performance of thermoelectric devices to a new dimension. Finally, we propose a self-powered single-dimension strain sensor based on thermoelectric power, reaching a gauge factor of ∼16.

Spatially-Resolved Study of the Electronic Transport and Resistive Switching in Polycrystalline Bismuth Ferrite

Ferroelectric materials attract much attention for applications in resistive memory devices due to the large current difference between insulating and conductive states and the ability of carefully controlling electronic transport via the polarization set-up. Bismuth ferrite films are of special interest due to the combination of high spontaneous polarization and antiferromagnetism, implying the possibility to provide multiple physical mechanisms for data storage and operations. Macroscopic conductivity measurements are often hampered to unambiguously characterize the electric transport, because of the strong influence of the diverse material microstructure. Here, we studied the electronic transport and resistive switching phenomena in polycrystalline bismuth ferrite using advanced conductive atomic force microscopy (CAFM) at different temperatures and electric fields. The new approach to the CAFM spectroscopy and corresponding data analysis are proposed, which allow deep insight into the material band structure at high lateral resolution. Contrary to many studies via macroscopic methods, postulating electromigration of the oxygen vacancies, we demonstrate resistive switching in bismuth ferrite to be caused by the pure electronic processes of trapping/releasing electrons and injection of the electrons by the scanning probe microscopy tip. The electronic transport was shown to be comprehensively described by the combination of the space charge limited current model, while a Schottky barrier at the interface is less important due to the presence of the built-in subsurface charge.

Recent progress of transport theory in Dirac quantum materials

Dirac quantum materials comprise a broad category of condensed matter systems characterized by low-energy excitations described by the Dirac equation. These excitations, which can manifest as either collective states or band structure effects, have been identified in a wide range of systems, from exotic quantum fluids to crystalline materials. Over the past several decades, they have sparked extensive experimental and theoretical investigations in various materials, such as topological insulators and topological semimetals. The study of Dirac quantum materials has also opened up new possibilities for topological quantum computing, giving rise to a burgeoning field of physics and offering a novel platform for realizing rich topological phases, including various quantum Hall effects and topological superconducting phases. Furthermore, the topologically non-trivial band structures of Dirac quantum materials give rise to plentiful intriguing transport phenomena, including longitudinal negative magnetoresistance, quantum interference effects, helical magnetic effects, and others. Currently, numerous transport phenomena in Dirac quantum materials remain poorly understood from a theoretical standpoint, such as linear magnetoresistance in weak fields, anomalous Hall effects in nonmagnetic materials, and three-dimensional quantum Hall effects. Studying these transport properties will not only deepen our understanding of Dirac quantum materials, but also provide important insights for their potential applications in spintronics and quantum computing. In this paper, quantum transport theory and quantum anomaly effects related to the Dirac equation are summarized, with emphasis on massive Dirac fermions and quantum anomalous semimetals. Additionally, the realization of parity anomaly and half-quantized quantum Hall effects in semi-magnetic topological insulators are also put forward. Finally, the key scientific issues of interest in the field of quantum transport theory are reviewed and discussed.

Electronic Band Structure of MXene Material (V2C)

MXene materials have received a strong boost of interest due to the prediction of behaving as topological dielectric or non conductor which is used in spintronic devices and for future technology device. In this experiment DFT (Density Functional Theory) based Quantum Express code is used to calculate the band structure and DOS of MXene material (i.e. V2C) by setting non-polarized spin polarization framework. From the geometric optimization, lattice parameter is found to be a = 2.945 Å, b = 8.836 Å, c = 28.19 Å. Since its bulk is insulating in nature, we observed MXene layer as conducting i.e. bands overlapping within fermi surface. Similar kind of scenario is observed in DOS which resembles escorted by the previously predicted results.

Fabrication of Sx/In2O3−x based microsphere for photoexcitation enhancing the NO2 gas detection properties

Development research as a NO2 gas sensor has attracted extensive attention recently. However, how to improve the electrical conductivity of materials, adjust the band structure of materials through photoexcitation and improve the carrier lifetime is still a challenge. In this work, an S-doped In2O3 structure (Sx/In2O3−x) was synthesized by enforcing the simple hydrothermal method. Doping directly controls the distribution of structural defects and oxygen vacancies in the material structure, and the response to NO2 increases by orders of magnitude under photoexcitation. Various structural characterization techniques examined the properties of the Sx/In2O3−x-based material. More specifically, its electro-optical properties and the impact of S doping on its energy band structure were thoroughly analyzed to investigate the origins of the enhanced responsivity.

DFT and AMPS-1D simulation analysis of all-perovskite solar cells based on CsPbI3/FAPbI3 bilayer structure

Very recently, perovskite solar cells (PSCs) have been proposed with both new device structure and materials to replace the conventional Methyl Ammonium-based structures in order to develop cells with higher operational stability. All-perovskite solar cells are the most emerging class of organic-inorganic PSCs which combine the photogeneration and absorption capability of two different perovskite materials to ensure both photo conversion performance and device operational stability. This simulation work aims to develop a systematic investigation of an all-perovskite solar cell structure made of CsPbI3/FAPbI3 heterojunctions using wxAMPS-1D platform supported with Density Functional Theory (DFT) simulations to analyze the band structure of both CsPbI3 and FAPbI3 materials. The quantum efficiency, device characteristics, and degradation trend have been optimized against the thickness of perovskite layers. The optimum thicknesses are obtained to be 200 nm for CsPbI3 layer at an assumed thickness range of∼300 nm for FAPbI3 as stated in different literature, 30 nm for TiO2 electron transporting layer, and 80 nm for Spiro-OMeTAD hole transporting layer. The optimized CsPbI3/FAPbI3 bilayer all-perovskite solar cell results in 19.94% efficiency (PCE), short-circuit current density (Jsc = 24.5 mA/cm2), and open circuit voltage (Voc = 1.1 V), and a fill factor (FF) = 74%. The bilayer structure (CsPbI3/FAPbI3) shows 20% improvement in external quantum efficiency (in visible range), compared to the cell made of single CsPbI3 or FAPbI3 layer. Moreover, the degradation analysis shows that increasing the mid-gap defect density (in both layers) up to 1015 cm−3 was detrimental to device performance.

Ytterbium doping reduces the dark current of UV photoelectric detector based on TiO2

Improved performance of TiO2-based UV photodetectors needs functional improvements in materials. Doping can modulate the electron and hole concentration and band structure of materials. This work shows that doping Yb into TiO2 improves the performance of TiO2-based UV photodetectors. It is found experimentally that doping Yb can cause Yb ions to replace Ti ions in lattice of TiO2, which is conducive to adjusting the physical and chemical properties from the perspective of energy band engineering. The dark current of the detector was less than 1 nA at bias of 5 V. Under UV radiation, the device achieved a maximum switching rate of 3.64 ⅹ 103 when the doping concentration of Yb was 5%, and the recovery time dropped from 1.4 s to 0.65 s.

About an (Im‐)Possible Anomaly in Band Structure Theory

Based on an envelope function approach pioneered by Wannier and Slater, it can be shown that under the influence of strong effective magnetic fields in spinor space the band structure of certain materials gives rise to the analogs of tardyons and luxons, and in principle also to the analogs of tachyons. Such a (hypothetical) tachyonic band structure would be metallic, but with a gap in k ‐space and not in the range of allowed band energies . Some of the conditions to observe such a tachyonic anomaly in certain types of electronic band structures, which do not seem to be implausible, are discussed. Similar anomalies should appear in the spectrum of Landau levels. Furthermore, some of the pathologies that would come with such exotic band structure anomalies are pointed out.

Tuning Band Gaps in Twisted Bilayer Borophene

Two-dimensional (2D) materials have captured considerable attention owing to their different properties compared to those of three-dimensional (3D) structures. The participation of rotation angles allows 2D bilayer materials to show more diverse physical properties. Here, we investigated the structural and electronic properties of bilayer borophene with four different rotation angles using first-principles calculations based on the density functional theory method. We found that the cohesive energy shows the same trend as the interlayer spacing, where the system with 83.1° is the most stable and its partial charge density shows the formation of apparent interlayer π-bonds. Meanwhile, the rotation angles alter the energy band structures and reduce the band gap sizes. It is worth noting that when 83.1°, a Dirac-like cone exists in Γ-Y. Finally, 31.4 and 83.1° bilayer borophene with initial band gaps exhibit the same band gap change behavior under external electric fields. These results extend the understanding of the interlayer interactions of twisted bilayer borophene with van der Waals interactions and provide a new and effective approach to precisely tune the band structure in bilayer materials.