Moderate Band Gap Research Articles

Mechanistic understanding of the interfacial properties of metal-PtSe2 contacts.

With the advantages of a moderate band gap, high carrier mobility and good environmental stability, two-dimensional (2D) semiconductors show promising applications in next-generation electronics. However, the accustomed metal-2D semiconductor contact may lead to a strong Fermi level pinning (FLP) effect, which severely limits the practical performance of 2D electronics. Herein, the interfacial properties of the contacts between a promising 2D semiconductor, PtSe2, and a sequence of metal electrodes are systematically investigated. The strong interfacial interactions formed in all metal-PtSe2 contacts lead to chemical bonds and a significant interfacial dipole, resulting in a vertical Schottky barrier for Ag, Au, Pd and Pt-based systems and a lateral Schottky barrier for Al, Cu, Sc and Ti-based systems, with a strong FLP effect. Remarkably, the tunneling probability for most metal-PtSe2 is significantly high and the tunneling-specific resistivity is two orders of magnitude lower than that of the state-of-the-art contacts, demonstrating the high efficiency for electron injection from metals to PtSe2. Moreover, the introduction of h-BN as a buffer layer leads to a weakened FLP effect (S = 0.50) and the transformation into p-type Schottky contact for Pt-PtSe2 contacts. These results reveal the underlying mechanism of the interfacial properties of metal-PtSe2 contacts, which is useful for designing advanced 2D semiconductor-based electronics.

Tuning ferroelectric photovoltaic performance in R3c-CuNbO3 through compressive strain engineering: a first-principles study.

Most ferroelectric oxides exhibit relatively wide bandgaps, which pose limitations on their suitability for photovoltaics application. CuNbO3 possesses potential ferroelectric properties with an R3c polar structure that facilitate the separation of charge carriers under illumination, promoting the generation of photovoltaic effects. The optical and ferroelectric properties of R3c-CuNbO3, as well as the effect of strain on the properties are investigated by first-principles calculation in this paper. The calculated results indicate that R3c-CuNbO3 possesses a moderate band gap to absorb visible light. The interaction of Cu-O and Nb-O bonds is considered to have a crucial role in the photovoltaic properties of CuNbO3, contributing to the efficient absorption of visible light. The bandgap of CuNbO3 becomes smaller and the density of states near the conduction and valence bands becomes relatively uniform in distribution under compressive conditions, which improves the photoelectric conversion efficiency to 29.9% under conditions of bulk absorption saturation. The ferroelectric properties of CuNbO3 are driven by the Nb-O bond interactions, which are not significantly weakened by the compressive strain. CuNbO3 is expected to be an excellent ferroelectric photovoltaic material by modulation of compressive strain due to the stronger visible light absorption and excellent ferroelectric behavior.

Diamond/c-BN van der Waals heterostructure with modulated electronic structures

The structural and electronic properties of (100), (110), and (111) diamond/cubic boron nitride (c-BN) heterostructures are systematically investigated by first principles calculation. The interface between diamond and c-BN shows the weak van der Waals interactions, which is confirmed by the interface distance and interface binding energy. The diamond/c-BN structures are the direct bandgap semiconductors with moderate bandgap values ranging from 0.647 eV to 2.948 eV. This work helps to promote the application of diamond in electronic and optoelectronic devices.

Constructing a Redox-Active Cu(I)-Pyridyltriazine Framework for Catalytic Photoreduction of Nitrobenzenes and Carboxylic Cyclization of Alkynol with CO2.

A redox-active metal-organic framework, Cu(I)-TPT, was synthesized by combination of Cu(I), the halogenoid cyanide group (CN), and redox-active organic bridging ligand 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT) into one single framework. Cu(I)-TPT displays a two-dimensional (2D) plane structure by 1D -Cu(I)-CN- chains connected with TPT ligands. Cu(I)-TPT exhibits intrinsic semiconductive features with a moderate bandgap energy (1.97 eV). Under irradiation, Cu(I)-TPT has an electrical conductivity of 2 × 10-7 S cm-1 in the presence of the sacrificial electron donor ethanol under the ambient test conditions, which is owing to the π-π stacking interactions between TPT moieties, the d-π conjugation between the Cu(I) ion and the CN ligands, and the permanent microporosity. Cu(I)-TPT displayed highly efficient hole-electron separation and ordered electron transfer, which is beneficial for the photoreduction of nitrobenzene. In addition, Cu(I)-TPT displays high efficiency in carboxylic cyclization of alkynol with CO2 because it possesses highly decentralized Cu(I) catalytic sites to the active C≡C bond of alkynol and affluent N atoms on the 2D sheets to facilitate the trapping and activation of CO2.

Ab-initio study of vacancy-defective aluminium nitride nanosheets as trifluoroacetonitrile gas sensor

Trifluoroacetonitrile (CF3CN) is a decomposition product from the promising dielectric heptafluoro-isobutyronitrile (C4F7N) under partial discharge process, so can be used to detect failures in gas-insulated switchgear. In this work, we study through density functional theory (DFT) calculations the adsorption of CF3CN on pristine aluminium nitride nanosheets (AlNNS) and defective Al-vacancy (vAl) and N-vacancy (vN) AlNNS, in order to explore the potential of these nanomaterials as sensors for this gas. Our energetic analysis shows negligible adsorption energy for the pristine surface but strong interactions can be observed between the molecule and the monovacancy-defected surfaces, with adsorption energy values estimated to be about −2.2 and −3.1 eV for CF3CN/vN-AlNNS and CF3CN/vAl-AlNNS, respectively, which suggests chemisorption processes. Furthermore, our geometric and bonding studies revealed the formation of new N—Al interactions between gas and monovacancy-defected surfaces, with serious distortions for the vAl-AlNNS nanosheet. The electronic density of states and band structure analysis reveals moderate band gap variations after gas adsorption. In contrast, significant changes in the work function and the magnetic moment of defective AlNNS were computed after gas exposition, for all optimized adsorption configurations. Therefore, based on our results, vacancy-defected AlNNS could be suggested as possible material for CF3CN gas sensing.

Investigation and understanding of the mechanical properties of MXene by high-throughput computations and interpretable machine learning

2D transition metal carbides, nitrides, and carbonitrides (MXenes) have become prominent in energy storage, catalysis, environmental and nanoelectronics applications. In addition to their outstanding merits of electrical conductivity and electrochemically activity, the mechanical properties play crucial roles in all these applications but are usually ignored by researchers. Only quite a few publications have studied their mechanical properties and these studies still only represent a limited range of predictions compared to the large family of MXenes. Utilizing high-throughput computations and data-driven methods, this work studies the tensile stiffness (E2D) and strength (σs) of 157 types of MXenes that could be potentially fabricated. We find MXenes with the tensile stiffness ranging from 81.71 to 561.4 N/m, while 42 structures show higher stiffness than the well-known graphene and transition metal dichalcogenide (TMDs) monolayers. The E2D strongly depends on the thickness, bond strength and surface terminations. Surprisingly, we find that the surface terminations can significantly improve the E2D, and the improvement can reach nearly 100%. By using the recently developed interpretable machine learning method, we obtain the analytical formula of E2D with efficient and physically interpretable descriptors that can predict the stiffness accurately. As for the σs, it significantly scales with E2D (σs≈E2D/β, β=10.449), and can be also improved by surface terminations as well as E2D. In addition, we also find a few of MXene semiconductors, although most of MXenes are metals. Moreover, the mechanical properties of these MXenes are better than the widely used monolayer semiconductor TMDs. These MXenes semiconductors with a moderate band gap may open a new avenue for fabricating next-generation 2D nanoscale electronics, such as all-in-one monolayer devices.

Resonant Grating-Enhanced Black Phosphorus Mid-Wave Infrared Photodetector.

Black phosphorus (BP) has emerged as a promising materials system for mid-wave infrared photodetection because of its moderate bandgap, high carrier mobility, substrate compatibility, and bandgap tunability. However, its uniquely tunable bandgap can only be taken advantage of with thin layer thicknesses, which ultimately limits the optical absorption of a BP photodetector. This work demonstrates an absorption-boosting resonant metal-insulator-metal (MIM) metasurface grating integrated with a thin-film BP photodetector. We designed and fabricated different MIM gratings and characterized their spectral properties. Then, we show that an MIM structure increased room temperature responsivity from 12 to 77 mA W-1 at 3.37 μm when integrated with a thin-film BP photodetector. Our results show that MIM structures simultaneously increase mid-wave infrared absorption and responsivity in a thin-film BP photodetector.

The SWSe-BP vdW Heterostructure as a Promising Photocatalyst for Water Splitting with Power Conversion Efficiency of 19.4.

Hydrogen generation by photocatalytic water splitting has drawn enormous research attention for converting sunlight and water into clean and green hydrogen fuel. However, the search for a high efficiency photocatalyst for water splitting is a key challenge. Two dimensional (2D) van der Waals (vdW) heterostructures as photocatalysts exhibit many advantages over the stacked original materials. In this article, we designed two novel 2D vdW heterostructures composed of WSSe and blue phosphorene (BP) monolayers, SWSe-BP and SeWS-BP, which are thermodynamically stable at room temperature. Using first-principles calculations, we found that the SWSe-BP vdW heterostructure can act as a potential photocatalyst for water splitting due to its robust stabilities, type-II band alignment, moderate bandgap, and suitable band edge positions for the redox reactions of water splitting, strong optical absorption, and excellent power conversion efficiency (PCE). Remarkably, the PCE of the SWSe-BP vdW heterostructure can achieve approximately 19.4% under a 3% biaxial tensile strain.

Band Structure Engineering of 2D Heterotriangulene Polymers by Incorporating Acetylenic Linkages for Photocatalytic Hydrogen Production

By performing first-principles calculations, we propose to introduce acetylenyl (−C≡C−) into the honeycomb kagome lattices of heterotriangulenes (HT)-based two-dimensional (2D) polymers to construct 2D HT-polyynes with tunable electronic and optical properties for photocatalysis. It is found that the band gap can be effectively reduced to a moderate value of around 2.00 eV by increasing the number of −C≡C– (n = 1, 2, 3) in the skeletons, contributing to enhanced light-harvesting ability in the visible range of the spectrum. Interestingly, the band edges of 2D HT-polyynes shift upward after incorporating with −C≡C– and then shift downward when the number of incorporated −C≡C– increases from 1 to 2 and 3. Possessing moderate band gap and matching band-edge alignments, high carrier mobility, and pronounced light-harvesting capability, the designed 2D HT-polyynes are predicted to be potential candidates for photocatalytic hydrogen production.

2D Boron Carbide, Carbon Nitride, and Silicon Carbide: A Theoretical Prediction

Two-dimensional (2D) boron carbide, carbon nitride, and silicon carbide are proposed with the aid of the density functional theory (DFT) simulations. These 2D systems are actually designed by doping the corresponding heteroatoms in the 2D Me-graphene (also called C568) system. Their structural stabilities are verified by analyzing their phonon relations. The mechanical strength of the Me-graphene-like silicon carbide (Me-C8Si4C) is weakened to some extent compared with the pristine Me-graphene, while the mechanical property of the proposed boron carbide (Me-C8B4C) is much enhanced. Moreover, according to our calculations at the HSE06 level, the Me-C8B4C system is metallic, while the proposed carbon nitride (Me-C8N4C) and silicon carbide (Me-C8Si4C) are semiconductors. Especially, the Me-C8N4C system exhibits a moderate band gap of 1.869 eV at the HSE06 level, which implies its application potentials in the solar conversion and photocatalysis fields. Furthermore, the influences of the biaxial strains on these designed systems are also discussed.

Sumanene Monolayer of Pure Carbon: A Two-Dimensional Kagome-Analogy Lattice with Desirable Band Gap, Ultrahigh Carrier Mobility, and Strong Exciton Binding Energy.

The design and synthesis of novel two-dimensional (2D) materials that possess robust structural stability and unusual physical properties may open up enormous opportunities for device and engineering applications. Herein, a 2D sumanene lattice that can be regarded as a derivative of the conventional Kagome lattice is proposed. The tight-binding analysis demonstrates sumanene lattice contains two sets of Dirac cones and two sets of flat bands near the Fermi surface, distinctively different from the Kagome lattice. Using first-principles calculations, two possible routines for the realization of stable 2D sumanene monolayers (named α phase and β phase) are theoretically suggested, and an α-sumanene monolayer can be experimentally synthesized with chemical vapor deposition using C21 H12 as a precursor. Small binding energies on Au(111) surface (e.g., -37.86eVÅ-2 for α phase) signify the possibility of their peel-off after growing on the noble metal substrate. Importantly, the GW plus Bethe-Salpeter equation calculations demonstrate both monolayers have moderate band gaps (1.94eV for α) and ultrahigh carrier mobilities (3.4 × 104 cm2 V-1 s-1 for α). In particular, the α-sumanene monolayer possesses a strong exciton binding energy of 0.73eV, suggesting potential applications in optics.

Thermodynamic Stability, Structure, and Optical Properties of Perovskite-Related CsPb2Br5 Single Crystals under Pressure

CsPb2Br5 belongs to all inorganic perovskite-related quasi-two-dimensional materials that have attracted considerable attention due to their potential for optoelectronic applications. In this study, we solve numerous controversies on the physical properties of this material. We show that optical absorption in the visible spectrum and green photoluminescence are due to microcrystallites of the three-dimensional CsPbBr3 perovskite settled on the CsPb2Br5 plates and that carefully cleaned crystal plates are devoid of these features. The high-pressure structural and spectroscopic experiments, performed on the single crystals free of CsPbBr3 impurities, evidenced that the layered tetragonal structure of CsPb2Br5 is stable at least up to 6 GPa. The absorption edge is located in the ultraviolet at around 350 nm and continuously red shifts under pressure. Moderate band gap narrowing is well correlated to the pressure-induced changes in the crystal structure. Although the compressibility of CsPb2Br5 is much higher than for CsPbBr3, the response in optical properties is weaker because the Pb–Br layers responsible for the optical absorption are much less affected by hydrostatic pressure than those built of Cs+ cations. Our study clarifies the confusing data in the literature on the optical properties and thermodynamic stability of CsPb2Br5.

Ab initio investigation of the structural and electronic properties of tantalum thallium chalcogenides TaTlX3 (X = S,Se)

The anisotropy of the structural and electronic parameters of the quasi-one-dimensional ternary transition metal chalcogenides TaTlX3(X ​= ​S, Se) is investigated through first principles calculations employing GGA-PBE and HSE06 functionals. The lattice parameters, bondlengths and cell volume obtained through GGA-PBE calculations are in excellent agreement with experimental results. Chains of edge sharing [TaX6]1∞ octahedral columns along [010] account for the quasi-one-dimensional structure of both compounds. Bandstructure calculations employing HSE06 functional show that TaTlS3 and TaTlSe3 are indirect bandgap semiconductors with bandgap of 1.15 ​eV and 0.84 ​eV respectively. The Q1D structure stems from the strong Ta-X bonds along the b-direction and weaker bonds in the other two directions, as reflected in the dispersion diagrams. Carrier effective mass and charge recombination rates show marked anisotropy along different high symmetry directions of the Brillouin zone, with significantly small values along the b-direction. Small carrier effective masses and charge recombination rates of the materials indicate their possible solar energy applications. Anisotropy of carrier mobility of these materials may make them suitable for novel sensor applications. The moderate bandgap of the materials are suitable for bottom layers of tandem solar cells.

First-principles study on ultralow lattice thermal conductivity in HfGeTe4

Materials with ultralow lattice thermal conductivity have attracted considerable interest in thermoelectrics and thermal management. Here, we have studied the lattice thermal conductivities in the layered material, hafnium germanium tetratelluride HfGeTe4, by the first-principles calculations and revealed an ultralow out-of-plane lattice thermal conductivity of [Formula: see text] at 300 K. HfGeTe4 also had a large anisotropy in its lattice thermal conductivities, and the [Formula: see text]/[Formula: see text] ratio reached as high as 10.4 at 300 K. By analyzing its phonon dispersion, group velocities, and lifetimes, we find that the ultralow lattice thermal conductivity of HfGeTe4 was mainly due to the small group velocities and short lifetimes of phonons, which was essentially caused by its heavy elements and complex layered crystal structure. Considering that HfGeTe4 is a semiconductor with a moderate band gap, HfGeTe4 would be a potential thermoelectric material. Our work sheds light on future studies on the thermoelectric properties of HfGeTe4.

Two novel easily exfoliated quaternary chalcogenides with high performance of photocatalytic hydrogen production

Based on density functional theory calculations, we have disclosed two novel monolayers Tl2SnAs2Q6 (Q = S, Se) with efficient photocatalytic ability. Both Tl2SnAs2S6 and Tl2SnAs2Se6 monolayers (referred to as TSAS and TSASe) show low cleavage energy comparable to that of graphene, which endorses they can be easily exfoliated from their bulk counterparts. Moderate band gaps are observed in TSAS (2.06 eV) and TSASe (1.81 eV) monolayers and both of their absorption peaks reach the order of 105 cm−1 in visible light region. Remarkably, the results of Gibbs free energy changes show that the hydrogen reaction can proceed efficiently without any extra voltage. Superior carrier mobilities with obvious anisotropy are also observed in TSAS and TSASe (∼103 cm2V-1s-1) monolayers, which are much larger than that of MoS2 (200 cm2V-1s-1). In addition, we also studied the adsorption energy of water molecules H2O and hydrogen atoms H on TSAS(Se) substrates, and the kinetic barriers in the process of hydrogen production, which is helpful to further understand the hydrogen evolution process of water splitting. This work demonstrates two novel easily exfoliated monolayers with significant hydrogen evolution performance, making them promising candidates for novel photocatalysts.

Point‐to‐face contact heterojunctions: Interfacial design of 0D nanomaterials on 2D g‐C3N4 towards photocatalytic energy applications

AbstractGreen energy generation is an indispensable task to concurrently resolve fossil fuel depletion and environmental issues to align with the global goals of achieving carbon neutrality. Photocatalysis, a process that transforms solar energy into clean fuels through a photocatalyst, represents a felicitous direction toward sustainability. Eco‐rich metal‐free graphitic carbon nitride (g‐C3N4) is profiled as an attractive photocatalyst due to its fascinating properties, including excellent chemical and thermal stability, moderate band gap, visible light‐active nature, and ease of fabrication. Nonetheless, the shortcomings of g‐C3N4 include fast charge recombination and limited surface‐active sites, which adversely affect photocatalytic reactions. Among the modification strategies, point‐to‐face contact engineering of 2D g‐C3N4 with 0D nanomaterials represents an innovative and promising synergy owing to several intriguing attributes such as the high specific surface area, short effective charge‐transfer pathways, and quantum confinement effects. This review introduces recent advances achieved in experimental and computational studies on the interfacial design of 0D nanostructures on 2D g‐C3N4 in the construction of point‐to‐face heterojunction interfaces. Notably, 0D materials such as metals, metal oxides, metal sulfides, metal selenides, metal phosphides, and nonmetals on g‐C3N4 with different charge‐transfer mechanisms are systematically discussed along with controllable synthesis strategies. The applications of 0D/2D g‐C3N4‐based photocatalysts are focused on solar‐to‐energy conversion via the hydrogen evolution reaction, the CO2 reduction reaction, and the N2 reduction reaction to evaluate the photocatalyst activity and elucidate reaction pathways. Finally, future perspectives for developing high‐efficiency 0D/2D photocatalysts are proposed to explore potential emerging carbon nitride allotropes, large‐scale production, machine learning integration, and multidisciplinary advances for technological breakthroughs.

Anisotropic interface characteristics of bilayer GeSe based field effect transistors

Moderate direct band gap and strong stability render bilayer gemanium selenide (GeSe) more suitable for the channel material in the electronic and optoelectronic devices. To improve carrier injection efficiency in an actual two dimensional device, the choice of the metal electrode is a key issue. Here using both electronic band calculations and quantum transport simulation, we comprehensively investigate the interface characteristics of bilayer GeSe contacted with six representative metals. The projected band structures of bilayer GeSe on these metals illustrate that all of these bilayer GeSe have undergone metallization, resulting in the contact natures are ohmic in the vertical interface. While，anisotropic lateral Schottky contacts are formed between the Ag, Au, Pd, Pt, Cu and bilayer GeSe along the armchair and the zigzag direction. The Ni electrode forms p-type quasi-Ohmic contact with bilayer GeSe with a small hole Schottky barrier height of 0.04 (0.08) eV along the armchair (zigzag) direction. Thus, the high performance of bilayer GeSe based FET would be realized by choosing the Ni as source/drain electrodes.

Enhanced photogalvanic effect in a 2D ferroelectric ZrI2 by interlayer sliding

Two-dimensional ferroelectrics with moderate band gap enable unprecedented applications in optoelectronics. Here, we report that the photogalvanic effect (PGE) of narrow-band-gap semiconductor bilayer ZrI2 is significantly enhanced by the interlayer sliding which turns out to be ferroelectric with both in-plane and out-of-plane polarizations. The intrinsic ferroelectric field promotes the separation efficiency of photogenerated carriers and reduces the recombination rate of electron–hole pairs, thus improving the photoelectric conversion efficiency and resulting in a larger photoresponse. The magnitude of the maximum photoresponse in the bilayer β-ZrI2 (ferroelectric) is enhanced by about 5 times than in the bilayer s-ZrI2 (paraelectric). A robust broadband photoresponse from mid-infrared to visible can be found from its photodetector. Our results provide great insights to engineering novel optoelectronic applications by ferroelectric slidetronics.

A DFT study of the electronic, optical, and mechanical properties of a recently synthesized monolayer fullerene network

Closely packed quasi-hexagonal and quasi-tetragonal crystalline phase of C60 molecules (named qHPC60) was recently synthesized. Here, we used GGA-PBE based DFT simulations to investigate the optoelectronic and mechanical properties of qHPC60 monolayers. qHPC60 has a moderate direct electronic bandgap, with anisotropic mechanical properties. Their elastic modulus ranges between 50–62 GPa. The results for optical properties suggest that qHPC60 can act as a UV collector for photon energies up to 5.5 eV since it presents low reflectivity and refractive index greater than one. The estimated optical bandgap (1.5–1.6 eV) is in very good agreement with the experimental one (1.6 eV).

Effect of metal doping in Bi2WO6 micro-flowers for enhanced photoelectrochemical water splitting

Design and fabrication of artificial catalytic systems to mimic natural photosynthesis which can harvest solar energy and directly convert into useable or storable energy resources may resolve the global energy crisis. Remarkably, photocatalysis is an efficient approach to utilize solar photons in order to drive the thermodynamic uphill reaction to generate chemical fuels in presence of efficient photocatalyst. Bismuth tungsten oxide (Bi2WO6) is a typical Aurivillius double perovskite oxides and has received an increasing research interest for water splitting due to moderate bandgap, superior chemical and thermal stability. However, the solar light absorption of bare Bi2WO6 is not significant in visible region, and low charge separation efficiency limits the photocatalytic performance. In the present work, in situ doping of flower like Bi2WO6 microstructure (∼6–8 μm) by metal (Mo, Fe, Zn) may improve the absorption and charge transfer efficiency. The metal doping effectively reduces the bandgap of Bi2WO6 which in turn enhances light absorption in the visible region. Notably, photoelectrochemical water splitting highly improved after metal doping, for example, Mo doping showed highest photocurrent density which is fifty-seven times higher than bare Bi2WO6. The band structure of Bi2WO6 before and after doping has been investigated on the basis of valence band XPS and Mott-Schottky analysis to understand the effect of doping in optoelectronic properties. The enhanced activity may originate due to presence of additional electronic states via doping, lower ionization potential, work function energy and enhanced number of charge carriers at the electrode-electrolyte interface during water splitting.