Tunable Electronic Structures Research Articles

Tunable electrical contact properties in two-dimensional van der Waals V2C/MoSi2N4 heterostructures

Two-dimensional MoSi2N4 is a member of the emerging 2D MA2N4 family, which has been synthesized in experiments, recently. Herein, we conduct a first-principles investigation to study more about the atomic and electronic structures of V2C/MoSi2N4 (1T-phase) van der Waals heterostructures (vdWHs) and interlayer distance and an external perpendicular electric field change their tunable electronic structures. We demonstrate that the V2C/MoSi2N4 vdWHs contact forms n-type Schottky contact with an ultralow Schottky barrier height of 0.17 eV, which is beneficial to enhance the charge injection efficiency. In addition, the electronic structure and interfacial properties of V2C/MoSi2N4 vdWHs can be transformed from n-type to p-type ShC through the effect of layer spacing and electric field. At the same time, the transition from ShC to OhC can also occur by relying on the electric field and different interlayer spacing. Our findings could give a novel approach for developing optoelectronic applications based on V2C/MoSi2N4 vdW heterostructures.

Two-Dimensional Metal-Organic Frameworks Towards Spintronics.

The intrinsic properties of predesignable topologies and tunable electronic structures, coupled with the increase of electrical conductivity, make two-dimensional metal-organic frameworks (2D MOFs) highly prospective candidates for next-generation electronic/spintronic devices. In this Minireview, we present an outline of the design principles of 2D MOF-based spintronics materials. Then, we highlight the spin-transport properties of 2D MOF-based organic spin valves (OSVs) as a notable achievement in the progress of 2D MOFs for spintronics devices. After that, we discuss the potential for spin manipulation in 2D MOFs with bipolar magnetic semiconductor (BMS) properties as a promising field for future research. Finally, we provide a brief summary and outlook to encourage the development of novel 2D MOFs for spintronics applications.

Two‐Dimensional Metal‐Organic Frameworks Towards Spintronics

AbstractThe intrinsic properties of predesignable topologies and tunable electronic structures, coupled with the increase of electrical conductivity, make two‐dimensional metal–organic frameworks (2D MOFs) highly prospective candidates for next‐generation electronic/spintronic devices. In this Minireview, we present an outline of the design principles of 2D MOF‐based spintronics materials. Then, we highlight the spin‐transport properties of 2D MOF‐based organic spin valves (OSVs) as a notable achievement in the progress of 2D MOFs for spintronics devices. After that, we discuss the potential for spin manipulation in 2D MOFs with bipolar magnetic semiconductor (BMS) properties as a promising field for future research. Finally, we provide a brief summary and outlook to encourage the development of novel 2D MOFs for spintronics applications.

Energy Storage Mechanism in Supercapacitors with Porous Graphdiynes: Effects of Pore Topology and Electrode Metallicity.

Porous graphdiynes are a new class of porous two-dimensional materials with tunable electronic structures and various pore structures. They have potential applications as well-defined nanostructured electrodes and could provide ideal platforms for understanding energy storage mechanisms underlying supercapacitors. Herein, we investigate the effect of stacking structure and metallicity on energy storage with such electrodes. Simulations revealed that supercapacitors based on porous graphdiynes of AB stacking structure could achieve both higher double-layer capacitance and ionic conductivity than AA stacking. This phenomenon is ascribed to more intense image forces in AB stacking, leading to a breakdown of ionic ordering and the formation of effective "free ions". Macroscale analysis shows that doped porous graphdiynes could deliver both outstanding gravimetric and volumetric energy and power densities due to their enhanced quantum capacitance. These findings pave the way for designing high-performance supercapacitors by regulating pore topology and metallicity of electrode materials. This article is protected by copyright. All rights reserved.

The effect of biaxial strain on the electronic structures and optical properties of GaS/SSnSe heterojunction: A first-principles calculations

Due to their tunable electronic structures and superior optical properties, vertically stacked heterojunctions have received considerable attention. In this paper, the electronic structure, charge transfer, and optical properties of the GaS/SSnSe heterojunction are investigated by first-principles calculations. According to our findings, the GaS/SSnSe heterojunction is a semiconductor with an indirect band gap and a Type-I band alignment. The band alignment of the heterojunction can be linearly tuned by the biaxial strain, which induces a semiconductor-to-metal transition. Moreover, the absorption coefficient of the heterojunction becomes higher and the absorption spectrum appears blue-shifted compared to each monolayer material. The above findings provide a theoretical support for the application of the heterojunction in ultraviolet light detection and tunable devices fields.

Local strain engineering in Janus MoSSe nanoribbons induces tunable electronic structures and remarkable magnetic moments

Local strain, as a small degree and single direction strain method, can effectively regulate the structures and electronic properties of armchair Janus MoSSe nanoribbon, so that the system can be transformed from the original 0.467 eV indirect band gap into 0.259 eV (3-zig), 0.117 eV (3-arm), 0.080 eV (6-arm) and 0.139 eV (9-zig) direct band-gap semiconductor according to the different strain degrees and directions. Compared with traditional MoS2 and MoSe2 nanoribbons, Janus MoSSe nanoribbon shows relatively stable band structure under local strain. The structure and electronic properties of Janus MoSSe nanoribbon are anisotropic when the local strain is along different directions. Due to the broken mirror symmetry of the Janus system and the appearance of in-plane local polarization, the spin polarization effect of Janus nanoribbon under local strain is more remarkable. When the local strain degree C = 0.167 is along the zigzag direction and the local strain C ⩾ 0.056 is along the armchair direction, the Janus nanoribbon exhibits half-metallic properties and surprisingly induces a magnetic moment. For the local strain along the armchair direction, the total magnetic moment of the system can be up to 2.05 μB when C = 0.111. A local strain method is applied to the nanoribbon system, which can effectively regulate the geometric configuration and electronic structure without external doping, and introduce magnetism, providing the possibility for expanding nanoribbons as potential nanoelectronic and spintronic materials.

Understanding remarkably high triplet quantum yield in thione analogs of perylenediimide: A detailed theoretical study.

The diverse and tunable electronic structures of perylenediimide (PDI), together with its high thermal and chemical stability, make the compound suitable for applications in bioimaging, electrical, and optical devices. However, a large singlet-triplet gap (ΔES-T) and almost zero spin-orbit coupling (SOC) between the lowest excited singlet (S1) and triplet (T1) restrict intersystem crossing (ISC) in highly fluorescent pristine PDI, yielding a near zero triplet quantum yield (ΦT). Interestingly, a thione analogs of PDI with varied S content (mS-PDIs, m = 1-4) was experimentally shown to yield ΦT ∼ 1.0 through efficient ISC. Time-dependent optimally-tuned range-separated hybrid calculations are performed to rationalize the experimentally observed red-shifted optical absorption and also the remarkably high ISC with almost zero radiative fluorescence reported for these mS-PDIs. To this end, the relative energies of low-lying excited singlets Sn (n = 1, 2) and a few triplets Tn(n = 1-3), along with their nature (nπ* or ππ*), are assessed for each of the mS-PDIs studied in chloroform. To our surprise and contrary to the earlier reports, both S1 and T1 are found to be of the same ππ* character, originating from the highest occupied to lowest unoccupied orbital transition, which, therefore, leads to a still large ΔES-T and vanishingly small SOC, as expected from the identical wavefunction symmetry. Increasing S content lowers S1(ππ*) due to a greater extent of π-delocalization, which well complements and supports the observed red-shift. More importantly, the T2 (or T3) closely lying to the S1 is of nπ* and, therefore, produces a relatively smaller ΔES-T and larger SOC. Detailed kinetics analysis suggests S1(ππ*) → T2(nπ*) is the primary ISC channel for all mS-PDIs, which is responsible for the remarkably high ΦT observed. In addition, comparable SOC and ΔES-T produce similar ISC rates for all mS-PDIs.

Hydrogenation-induced ultrahigh stabilization and tunable electronic structures of two-dimensional orthorhombic diboron diphosphide

Using density-functional theory calculations, we have studied hydrogenated two-dimensional (2D) orthorhombic diboron diphosphorus (O-B2P2). It is found that hydrogenation can transit the pristine O-B2P2 from a tiny bandgap semiconductor to a wide- and indirect-bandgap semiconductor, and the bandgaps are dependent on hydrogenation configurations. Moreover, our calculations have revealed that the three hydrogenated O-B2P2 nanostructures are both dynamically and thermally stable, and their bandgaps are estimated to be 2.8–4.2 eV according to hybrid potential calculations. They are predicted to possess strongly anisotropic mechanical and carrier transport properties, allowing potential applications for in-plane anisotropic and high-performance electronic devices. Hydrogenated O-B2P2 nanostructures exhibit strong absorbance of ultraviolet light and their bandgaps can be linearly modulated by tensile strain. Our findings demonstrate novel mechanical and electronic properties of hydrogenated O-B2P2 nanostructures, combined with excellent stability in ambient conditions, suggesting that they could be promising candidates for strongly anisotropic electronic and sensor devices.

Robust Electronic Structure: Uncovering the Origins of Fast Oxygen Reduction Kinetics of the NiCo Nanoalloys@N-Doped Carbon

The non-noble nanoalloy family is one of the promising catalysts for oxygen reduction reactions and zinc–air batteries. However, the complex reconstruction behavior is not clear enough for guiding the design of alloy catalysts. The origins of reaction kinetics during drastic aging require further investigation. Hence, we prepare several NiCo nanoalloys@NC with tunable electronic structures. Among them, Ni2Co4@NC displays a suitable electronic structure, in which the interacted Ni and Co sites accelerate oxygen reduction and evolution reactions. It delivers an ultralow Tafel slope of 46.3 mV dec–1 for the oxygen reduction reaction and 65.0 mV dec–1 for the oxygen evolution reaction and releases an excellent power density of 162.9 mW cm–2 in the zinc–air battery. With ex situ Raman and other characterizations, we subsequently deduce the reconstruction behavior of these NiCo nanoalloys@NC samples. The suitable surface oxyhydroxide–hydroxide–oxide shell accounts for the excellent stability and reaction kinetics of Ni2Co4@NC. With density functional theory simulations, we further discover its robust electronic structure during the drastic reconstruction so that it displays rapid kinetics after aging. In the experiment, its oxygen reduction reaction (ORR) Tafel slope merely increased from 46.3 to 48.6 mV dec–1. We highlight the decisive role of the surface electronic structure in electrochemical kinetics and explain how to achieve excellent stability.

Multi-functional MXene quantum dots enhance the quality of perovskite polycrystalline films and charge transport for solar cells

Recently, two-dimensional (2D) transition metal carbides/nitrides (MXenes) find applications in perovskite solar cells (PSCs), due to their high conductivity, tunable electronic structures, and rich surface chemistry, etc. However, the integration of 2D MXenes into PSCs is limited by their large lateral sizes and relatively-small surface volume ratios, and the roles of MXenes in PSCs are still ambiguous. In this paper, zero-dimensional (0D) MXene quantum dots (MQDs) with an average size of 2.7 nm are obtained through clipping step by step combining a chemical etching and a hydrothermal reaction, which display rich terminals (i.e., -F, –OH, -O) and unique optical properties. The 0D MQDs incorporated into SnO2 electron transport layers (ETLs) of PSCs exhibit multifunction: 1) increasing the electrical conductivity of SnO2, 2) promoting better alignments of energy band positions at the perovskite/ETL interface, 3) improving the film quality of atop polycrystalline perovskite. Particularly, the MQDs not only tightly bond with the Sn atom for decreasing the defects of SnO2, but also interact with the Pb2+ of perovskite. As a result, the defect density of PSCs is significantly decreased from 5.21 × 1021 to 6.4 × 1020 cm-3, leading to enhanced charge transport and reduced nonradiative recombination. Furthermore, the power conversion efficiency (PCE) of PSCs is substantially improved from 17.44% to 21.63% using the MQDs-SnO2 hybrid ETL compared with the SnO2 ETL. Besides, the stability of the MQDs-SnO2-based PSC is greatly enhanced, with only ～4% degradation of the initial PCE after storage in ambient condition (25 °C, RH: 30-40%) for 1128 h, as compared to that of the reference device with a rapid degradation of ～60% of initial PCE after 460 h. And MQDs-SnO2-based PSC also presents higher thermal stability than SnO2-based device with continuous heating for 248 h at 85 °C. The unique MQDs exhibited in this work might also find other exciting applications such as light-emitting diodes, photodetectors, and fluorescent probes.

Heteroatom‐ and Bonded Z‐Scheme Channels‐Modulated Ultrafast Carrier Dynamics and Exciton Dissociation in Covalent Triazine Frameworks for Efficient Photocatalytic Hydrogen Evolution

AbstractCovalent triazine frameworks (CTF) offer a tunable platform for photocatalytic H2 generation due to their diverse structures, low costs, and precisely tunable electronic structures. However, high exciton binding energy and short lifetimes of photogenerated carriers restrict their application in photocatalytic hydrogen evolution. Herein, a novel phosphorus‐incorporated CTF is introduced to construct a chemically bonded PCTF/WO3 (PCTFW) heterostructure with a precise interface electron transfer channel. The phosphorus incorporation is found to dominantly reduce the exciton binding energy and promote the dissociation of singlet and triplet excitons into free charge carriers due to the regulation of electronic structures. High‐quality interfacial WN bonds improve the interfacial transfer of photogenerated electrons, thus prolonging the lifetime of photogenerated electrons. Femtosecond transient absorption spectroscopy characterizations and DFT calculations further confirm both phosphorus incorporation and Z‐scheme heterojunctions can synergistically boost the in‐built electric field and accelerate the migration and separation of photogenerated electrons. The optimized photocatalytic H2‐evolution rate of resultant PCTFW is 134.84 µmol h−1 (67.42 mmol h−1g−1), with an apparent quantum efficiency of 37.63% at 420 nm, surpassing many reported CTF‐based photocatalysts so far. This work highlights the significance of atom‐level interfacial exciton dissociation, and charge transfer and separation in improving photocatalysis.

Hybrid Nanostructured Materials as Electrodes in Energy Storage Devices

The global demand for energy is constantly rising, and thus far, remarkable efforts have been put into developing high-performance energy storage devices using nanoscale designs and hybrid approaches. Hybrid nanostructured materials composed of transition metal oxides/hydroxides, metal chalcogenides, metal carbides, metal–organic frameworks, carbonaceous compounds and polymer-based porous materials have been used as electrodes for designing energy storage systems such as batteries, supercapacitors (SCs), and so on. Different kinds of hybrid materials have been shown to be ideal electrode materials for the development of efficient energy storage devices, due to their porous structures, high surface area, high electrical conductivity, charge accommodation capacity, and tunable electronic structures. These hybrid materials can be synthesized following various synthetic strategies, including intercalative hybridization, core–shell architecture, surface anchoring, and defect control, among others. In this study, we discuss applications of the various advanced hybrid nanostructured materials to design efficient batteries and SC-based energy storage systems. Moreover, we focus on their features, limitations, and real-time resolutions.

High‐rate CO2‐to‐CH4 Electrosynthesis by Undercoordinated Cu Sites in Alkaline‐Earth‐Metal Perovskites with Strong Basicity

AbstractThe electrochemical CO2 reduction to CH4 has been extensively demonstrated, but still suffers from relatively poor activity and requires high overpotentials especially at large electrolysis rates. Perovskite oxides (AxByO) are one type of promising electrocatalyst for the CO2 reduction due to their tunable electronic structures. In this work, a Ca2CuO3 perovskite oxide catalyst is developed with alkaline‐earth A‐sites, featuring an inherently strong basic strengthand outstanding capability for CO2 adsorption, as well as the undercoordinated Cu sites generated through partial surface Ca2+ cation leaching. The Ca2CuO3 catalyst exhibitsa high partial current density of 517 ± 23 mA cm−2 for producing CH4 at a low applied potential of −0.30 V versus reversible hydrogen electrode, which further reached to a peak value of 1452 ± 156 mA cm−2. Density functional calculations show that the undercoordinated Cu sites allowed to promote the hydrogenation of *CO and subsequent *CHO intermediates, thus leading to the high CH4 activity. This work suggests an attractive design strategy for tuning the A‐sites in perovskite oxides to realize high‐rate CO2‐to‐CH4 electrosynthesis with low overpotentials.

A thienyl‐benzodithiophene‐based two‐dimensional conjugated covalent organic framework for fast photothermal conversion

AbstractTwo‐dimensional conjugated covalent organic frameworks (2D c‐COFs) are emerging semiconductor materials for optoelectronic and photothermal applications. In particular, the highly tailorable, semiconducting thiophene‐based 2D c‐COFs have attracted considerable interest due to their nontrivial physicochemical properties such as photo‐activity, broad optical absorption, tunable electronic structures, and so forth. Herein, we demonstrate a novel, crystalline 2D c‐COF based on thienyl‐functionalized benzodithiophene (BDT) and biphenyl (BP) via the Schiff‐base polycondensation reaction. The resultant BDT‐BP‐COF exhibits a broad optical absorption up to ca. 600 nm and decent π‐conjugation along the 2D polymer skeleton, as revealed by the optical absorption and theoretical calculations. The favorable π‐conjugation and the abundant electron‐rich thiophene units confer excellent photo‐activity to BDT‐BP‐COF towards the usage of solar energy. As a proof‐of‐concept application, we explore BDT‐BP‐COF in photothermal conversion, in which it shows a fast surface‐temperature increase upon light irradiation for seconds.

Investigation of non-stoichiometric delafossite photoelectrodes composed of cuprous oxide and iron oxide for enhanced water-splitting

Photoelectrochemical energy conversion is the ultimate approach for converting sunlight energy into value-added chemical fuels. The path of achieving economically feasible, highly efficient, and earth-abundant material-based photoelectrodes can be a driving force for realizing PEC on a global scale. Copper-based delafossite photoelectrodes are attractive candidates because they exhibit appropriate absorption properties and structural stability in an aqueous system. In this study, delafossite CuxFe2−xO2 films of varying compositions ranging from 0 ≤ x ≤ 2 are prepared by adjusting the Cu/Fe composition via radio frequency plasma co-sputtering to evaluate the fundamental optoelectronic properties. The optical bandgap of the CuFeO2 films tuned from 1.4 to 1.65 eV, with the maximum carrier concentration of 5.9 × 1017 cm−3 noticed in Cu1.5Fe0.5O2 film. The X-ray photoelectron spectroscopy measurements confirm the existence of a high level of oxygen-deficient sites in CuFeO2 film photoelectrodes that are suitable for PEC performance. The fabricated non-stoichiometric photoelectrode exhibits an excellent hydrogen evolution reaction of 4.3 μmol/hcm2, nearly 1.8 times improvement over Cu2O photoelectrode with a long-term photostability even in an aqueous system. The ability to fabricate tunable electronic structures photoelectrodes along with oxygen-deficient surface defects facilitates to localize band edge closer to the HER of water, and the Fermi level is slightly away from the reduction potential of Cu2O, which supports to improve PEC performance by suppressing photo-induced corrosion to retain greater photostability.

Review on CeO2-Based Photocatalysts for Photocatalytic Reduction of CO2: Progresses and Perspectives

A promising approach is to transform carbon dioxide (CO2) into renewable, high value-added products with minimal environmental impact and maximum efficiency. Many problems, including energy shortages and environmental degradation, are caused by high CO2 levels, but this can help. Photocatalytic reduction methods are a promising technology for solving these problems. Cerium dioxide (CeO2) is one of the most important rare earth oxides, and cerium dioxide-based photocatalysts have gained a lot of attention in recent years due to their distinctive features, such as their tunable electronic structures and their excellent photocatalytic performances. However, the wide energy band of cerium oxide limits its utilization of light. This review provides various strategies for the modification of cerium oxide, encompassing external atom doping, heterostructure fabrication, defect fabrication, and crystal plane modulation, to improve the catalytic performance of CeO2. The fundamentals of the conversion of CeO2-based photocatalysts into high value-added products are also summarized. Finally, the challenges and prospects of CeO2-based catalysts for photocatalytic reduction of CO2 are presented.

MOF-Derived CoNi Nanoalloy Particles Encapsulated in Nitrogen-Doped Carbon as Superdurable Bifunctional Oxygen Electrocatalyst

Carbon-encapsulated transition metal catalysts have caught the interest of researchers in the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) due to their distinctive architectures and highly tunable electronic structures. In this work, we synthesized N-doped carbon encapsulated with CoNi nanoalloy particles (CoNi@NC) as the electrocatalysts. The metal-organic skeleton ZIF-67 nanocubes were first synthesized, and then Ni2+ ions were inserted to generate CoNi-ZIF precursors by a simple ion-exchange route, which was followed by pyrolysis and with urea for the introduction of nitrogen (N) at a low temperature to synthesize CoNi@NC composites. The results reveal that ZIF-67 pyrolysis can dope more N atoms in the carbon skeleton and that the pyrolysis temperature influences the ORR and OER performances. The sample prepared by CoNi@NC pyrolysis at 650 °C has a high N content (9.70%) and a large specific surface area (167 m2 g-1), with a positive ORR onset potential (Eonset) of 0.89 V vs. RHE and half-wave potential (E1/2) of 0.81 V vs. RHE in 0.1 M KOH, and the overpotential of the OER measured in 1 M KOH was only 286 mV at 10 mA cm-2. The highly efficient bifunctional ORR/OER electrocatalysts synthesized by this method can offer some insights into the design and synthesis of complex metal-organic frameworks (MOFs) hybrid structures and their derivatives as functional materials in energy storage.

Rational Design of Atomically Dispersed Metal Site Electrocatalysts for Oxygen Reduction Reaction.

Future renewable energy supply and a cleaner Earth greatly depend on various crucial catalytic reactions for the society. Atomically dispersed metal site electrocatalysts (ADMSEs) have attracted tremendous research interest and are considered as the next-generation promising oxygen reduction reaction (ORR) electrocatalysts due to the maximum atom utilization efficiency, tailorable catalytic sites, and tunable electronic structures. Despite great efforts have been devoted to the development of ADMSEs, the systematic summary for design principles of high-efficiency ADMSEs is not sufficiently highlighted for ORR. In this review, the authors first summarize the fundamental ORR mechanisms for ADMSEs, and further discuss the intrinsic catalytic mechanism from the perspective of theoretical calculation. Then, the advanced characterization techniques to identify the active sites and effective synthesis methods to prepare catalysts for ADMSEs are also showcased. Subsequently, a special emphasis is placed on effective strategies for the rational design of the advanced ADMSEs. Finally, the present challenges to be addressed in practical application and future research directions are also proposed to overcome the relevant obstacles for developing high-efficiency ORR electrocatalysts. This review aims to provide a deeper understanding for catalytic mechanisms and valuable design principles to obtain the advanced ADMSEs for sustainable energy conversion and storage techniques.

Density Functional Theory Combined with Thermodynamics Exploration of Novel 2D Materials Created Using Aqueous Exfoliation

During the past decade, two-dimensional (2D) nanomaterials have provided a broad platform for a wide variety of applications and have been the focus of numerous studies due to their unique properties. The most common 2D materials are of hexagonal or trigonal symmetry, with structures that can be grown on inexpensive substrates like Si and sapphire. The ability to grow 2D materials on additional substrates with a larger variety of symmetries and surface terminations, such as metals, glass, and complex metal oxides, can be regarded as a big and necessary step in the design of new functional materials. However, there is a dearth of knowledge on 2D materials beyond hexagonal and trigonal symmetries, their synthesis, and epitaxy. Here, we present a procedure to rationalize the aqueous exfoliation of known 3D materials as a route to create stable tetragonal 2D materials with tunable lattice parameters and electronic structures. We develop a model using first-principles density functional theory plus thermodynamics to propose an ecofriendly synthesis of 2D materials using aqueous media and use it to investigate a class of previously synthesized tetragonal ABX compounds. Then, we predict the thermodynamics of their exfoliation in water by combining surface slab models, their subsequent surface transformations, and experimentally determined changes in free energy. The atomistic results obtained here are correlated to periodic properties and are used to elucidate trends in electronic structure and the thermodynamics of aqueous exfoliation.

Understanding photocatalytic hydrogen peroxide production in pure water for benzothiadiazole-based covalent organic frameworks

In this study, we introduce two benzothiadiazole-based COFs with tunable electronic structures and electron affinities for photocatalytic H2O2 production.