Tunable band alignment and large power conversion efficiency in a two-dimensional InS/ZnIn2S4 heterostructure

Heterostructures can efficiently modulate the bandgap of semiconductors and enhance the separation of photocarriers, thereby enhancing the performance of optoelectronic devices. Herein, we design an InS/ZnIn2S4 van der Waals (vdW) heterostructure and investigate its electronic and photovoltaic properties using first principles calculation. Compared to its individual monolayers, the InS/ZnIn2S4 heterostructure not only possesses a smaller band gap of 2.21 eV and superior light absorption performance in the visible short-wavelength region (<500 nm) but also forms a type-II1 band alignment. Moreover, a large power conversion efficiency (PCE) of 10.86% is achieved. The transformation of the band alignment from type-II1 to type-I or type-II2 can be forced using an external electric field, and the PCE can be further increased up to 12.19% at a positive E ⊥ of 0.2 V Å-1. Within a critical biaxial strain of 4%, the type-II1 band alignment can be maintained, and a high PCE of 20.80% is achieved at a tensile strain (ε) of 4%. Our results may suggest a potential optoelectronic application direction for the InS/ZnIn2S4 heterostructure and offer effective means to enhance its optoelectronic device performance.

The tunneling electroresistance effect in a van der Waals ferroelectric tunnel junction based on a graphene/In2Se3/MoS2/graphene heterostructure

In recent years, α-In2Se3 has attracted great attention in miniaturizing nonvolatile random memory devices because of its room temperature ferroelectricity and atomic thickness. In this work, we construct two-dimensional (2D) van der Waals (vdW) heterostructures α-In2Se3/MoS2 with different ferroelectric polarization and design a 2D graphene (Gr)/In2Se3/MoS2/Gr ferroelectric tunnel junction (FTJ) with the symmetric electrodes. Our calculations show that the band alignment of the heterostructures can be changed from type-I to type-II accompanied by the reversal of the ferroelectric polarization of In2Se3. Furthermore, the ferroelectricity persists in Gr/In2Se3/MoS2/Gr vdW FTJs, and the presence of dielectric layer MoS2 in the FTJs enables the effective modulation of the tunneling barrier by altering the ferroelectric polarization of α-In2Se3, which results in two distinct conducting states denoted as "ON" and "OFF" with a large tunneling electroresistance (TER) ratio exceeding 105%. These findings suggest the importance of ferroelectric vdW heterostructures in the design of FTJs and propose a promising route for applying the 2D ferroelectric/semiconductor heterostructures with out-of-plane polarization in high-density ferroelectric memory devices.

Effect of 10 MeV electron irradiation on the electrical properties of bulk α-In2Se3 crystals

In this work, the effect of 10 MeV electron irradiation on the structure and electrical properties of bulk α-In2Se3 crystals is studied by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray microanalysis, atomic-force microscopy, and Raman spectroscopy methods. Droplets of 200-500 nm in size were detected on the bulk α-In2Se3 crystal surface. The droplets can be formations with the γ-In2Se3 crystalline phase. The current-voltage characteristics measured by conductive atomic-force microscopy are different on and outside the droplets after electron irradiation. On the droplets, slightly better conductive properties were detected after irradiation with the electron fluence of 1015 cm-2. It is found that local resistance increases significantly for both on and outside the droplets after irradiation with the electron fluence of 1017 cm-2. Our study shows that electron irradiation can contribute to the disappearance of ferroelectric domains in the bulk α-In2Se3 crystals. Also, the distribution of surface potentials measured by Kelvin probe force microscopy becomes more uniform after electron irradiation. The results obtained in the work allow us to calculate the operating time of the device containing α-In2Se3 under conditions of long-term electron irradiation with high-energy electrons. The study shows that α-In2Se3 is a very promising material for applications in the aerospace and nuclear industries.

First-principles study on photoelectric properties of all-inorganic two-dimensional double perovskite Cs3AgBiBr7

Two-dimensional (2D) all-inorganic double perovskite materials have attracted great interest owing to their unique photoelectric characteristics, such as high quantum efficiency and relative stability. However, few studies have been conducted on the 2D all-inorganic double perovskite Cs₃AgBiBr₇, and its photoelectric properties are unclear. In this study, we present a detailed investigation of the band structure, optical absorption spectrum, carrier mobility and exciton binding energy of the double perovskite Cs₃AgBiBr₇ based on the first-principles. The results show that this system has an indirect band gap and low carrier mobility, high exciton binding energy (2041.38 meV) and significant light absorption in the UV region. We also find that the material may be a potential exciton insulation candidate owing to the exciton binding energy beyond the band gap. Our calculated results also show that low dimensional perovskite Cs₃AgBiBr₇ is more suitable for luminescence than a photovoltaic device. We hope our theoretical results will inspire and promote the experimental exploration of 2D all-inorganic double perovskite materials for photoelectric applications.

Flexible Optical Synapses Based on In2Se3/MoS2 Heterojunctions for Artificial Vision Systems in the Near-Infrared Range

Near-infrared (NIR) synaptic devices integrate NIR optical sensitivity and synaptic plasticity, emulating the basic biomimetic function of the human visual system and showing great potential in NIR artificial vision systems. However, the lack of semiconductor materials with appropriate band gaps for NIR photodetection and effective strategies for fabricating devices with synaptic behaviors limit the further development of NIR synaptic devices. Here, a two-terminal NIR synaptic device consisting of the In₂Se₃/MoS₂ heterojunction has been constructed, and it exhibits fundamental synaptic functions. The reduced band gap and potential barrier of In₂Se₃/MoS₂ heterojunctions are essential for NIR synaptic plasticity. In addition, the NIR synaptic properties of In₂Se₃/MoS₂ heterojunctions under strain have been studied systematically. The ΔEPSC of the In₂Se₃/MoS₂ synaptic device can be improved from 38.4% under no strain to 49.0% under a 0.54% strain with a 1060 nm illumination for 1 s at 100 mV. Furthermore, the artificial NIR vision system consisting of a 10 × 10 In₂Se₃/MoS₂ device array has been fabricated, exhibiting image sensing, learning, and storage functions under NIR illumination. This research provides new ideas for the design of flexible NIR synaptic devices based on 2D materials and presents many opportunities in artificial intelligence and NIR vision systems.

Theoretical study on photocatalytic performance of ZnO/C2N heterostructure towards high efficiency water splitting

The construction of van der Waals heterostructures offers effective boosting of the photocatalytic performance of two-dimensional materials. In this study, which uses the first-principles method, the electronic and absorptive properties of an emerging ZnO/C₂N heterostructure are systematically explored to determine the structure’s photocatalytic potential. The results demonstrate that ZnO and C₂N form a type-II band alignment heterostructure with a reduced band gap, and hence superior absorption in the visible region. Furthermore, the band edge positions of a ZnO/C₂N heterostructure meet the requirements for spontaneous water splitting. The ZnO/C₂N heterostructure is known to possess considerably improved carrier mobility, which is advantageous in the separation and migration of carriers. The Gibbs free energy calculation confirms the high catalytic activity of the ZnO/C₂N heterostructure for water-splitting reactions. All the aforementioned properties, including band gap, band edge positions, and optical absorption, can be directly tuned using biaxial lateral strain. A suitable band gap, decent band edge positions, high catalytic activity, and superior carrier mobility thus identify a ZnO/C₂N heterostructure as a prominent potential photocatalyst for water splitting.

Ni-MOL/In2Se3 heterostructure construction with mixed metal (Ti/Ni) for efficient photocatalytic tetracycline degradation

To investigate the mechanism of bimetallic 2-dimension (2D) catalyst existing in the current photocatalytic degradation process, the tetracycline (TC) degradation performance and mechanism by bimetallic 2D photocatalyst was studied extensively. Nickel metal-organic layer (Ni-MOL) and In₂Se₃, a typical 2D semiconductor photocatalyst, shows great potential for photocatalytic degradation of TC. Herein, an In₂Se₃ assisted Ni-MOL composite bimetallic photocatalyst was assembled, of which could obtain the degradation rate of 96.4% within 90 min for TC under visible light. Ni-MOL was the main active site for TC degradation by photo-induced holes which located at the Ni atom active site during the photocatalytic process. The role of In₂Se₃ and the element of Ti in Ni-MOL was to assist Ni-MOL by providing more photo-induced carriers and inhibiting carrier recombination. This work makes a contribution to the application of 2D bimetallic photocatalytic in TC degradation.

2D and 3D double perovskite with dimensionality-dependent optoelectronic properties: first-principle study on Cs2AgBiBr6and Cs4AgBiBr8

Recently, the effect of dimensional control on the optoelectronic performance of two-dimensional (2D)/three-dimensional (3D) single perovskites has been confirmed. However, how the dimensional change affects the photoelectric properties of 2D/3D all-inorganic double perovskites remains unclear. In this study, we present a detailed theoretical research on a comparison between the optoelectronic properties of 3D all-inorganic double perovskite Cs₂AgBiBr₆and recently reported 2D all-inorganic double perovskite Cs₄AgBiBr₈with Ruddlesden-Popper (RP) structure based on density functional theory calculations. The results demonstrate the charge carrier mobility and absorption coefficients in the visible spectrum of Cs₄AgBiBr₈(2D) is poorer than Cs₂AgBiBr₆(3D). Moreover, the value of exciton-binding energy for 2D RP all-inorganic double perovskite Cs₄AgBiBr₈(720 meV) is 3 times larger than that of 3D all-inorganic double perovskite Cs₂AgBiBr₆(240 meV). Our works indicate that Cs₄AgBiBr₈(2D) is a promising material for luminescent device, while Cs₂AgBiBr₆(3D) may be suitable for photovoltaic applications. This study provides a theoretical guidance for the understanding of 2D RP all-inorganic double perovskite with potential applications in photo-luminescent devices.

High-performance photovoltaic application of the 2D all-inorganic Ruddlesden-Popper perovskite heterostructure Cs2PbI2Cl2/MAPbI3

The three-dimensional (3D) organic-inorganic halide perovskite MAPbI₃ has excellent light-harvesting properties but is unstable. However, the newly synthesized two-dimensional (2D) all-inorganic Ruddlesden-Popper (RP) perovskite Cs₂PbI₂Cl₂ has superior stability but adverse photoelectric properties. Therefore, constructing a 2D Cs₂PbI₂Cl₂/3D MAPbI₃ heterostructure is expected to combine the superstability of the 2D material and the high efficiency of the 3D one. The photoelectric properties and charge transfer of 2D Cs₂PbI₂Cl₂/3D MAPbI₃ heterostructures are investigated using density functional theory, where MAPbI₃ has two kinds of contacting interfaces, i.e., MAI and PbI interfaces. The band gaps of 2D/MAI and 2D/PbI heterostructures are 1.52 eV and 1.40 eV, smaller than those of the free-standing materials (2D ∼ 2.50 eV, MAI ∼ 1.77 eV, and PbI ∼ 1.73 eV), which can broaden the light absorption spectrum. Moreover, the 2D/3D heterostructures are typical type-II heterostructures, which is beneficial to facilitate the separation of carriers for increasing the photoelectric conversion. Interestingly, due to the work function difference (2D ∼ 4.97 eV, MAI ∼ 3.57 eV, and PbI ∼ 5.49 eV), the charge transfer directions of the 2D/MAI and 2D/PbI heterostructures are completely opposite, which shows that interface engineering to impose a consistent interface termination is needed to obtain good performance for solar cells. These results demonstrate that constructing 2D Cs₂PbI₂Cl₂ and 3D MAPbI₃ heterostructures by interfacial engineering is a potential strategy to improve the performance of perovskite solar cells (PSCs).

Effects of doping on photocatalytic water splitting activities of PtS2/SnS2 van der Waals heterostructures

Photocatalytic water splitting is a promising technology to solve serious energy and environmental problems. The PtS₂ monolayer has been previously predicted to be a water splitting photocatalyst. But the high efficiency of carrier recombination in the monolayer results in poor photocatalytic performance. It is well known that the construction of van der Waals (vdW) heterojunctions can improve the photocatalytic performance of a monolayer. In this investigation, we constructed a PtS₂/SnS₂ vdW heterojunction and systematically investigated the influence of the doping position and doping ratio on its performance using density functional theory calculations. Interestingly, the band alignment transforms from Type-II to Type-I and from Type-I to Type-II when the S in SnS₂ is replaced with Se in the PtS₂/SnS₂ vdW heterojunction and the S in PtS₂ is replaced with Se in the PtS₂/SnSe₂ vdW heterojunction, respectively. More importantly, from the PtS₂/SnS₂ to PtSe₂/SnSe₂ vdW heterojunction, the decomposition of water also changes from semi-decomposed water to fully decomposed water. Furthermore, the results show that the direct Z-scheme photocatalytic mechanism exists in the PtSSe/SnSe₂ vdW heterojunction by analysis of the migration paths of photoinduced electrons and holes. And compared with the PtS₂/SnS₂, the PtSSe/SnSe₂ heterostructure exhibits better photocatalytic water splitting activities. These results can provide a direction that doping can improve the photocatalytic water splitting performance of heterojunction photocatalysts.

Transition of the Type of Band Alignments for All-Inorganic Perovskite van der Waals Heterostructures CsSnBr3/WS2(1-x)Se2x

In general, two-dimensional semiconductor-based van der Waals heterostructures (vdWHs) can be modulated to achieve the transition of band alignments (type-I, type-II, and type-III), which can be applied in different applications. However, it is rare in three-dimensional perovskite-based vdWHs, and it is challenging to achieve the tunable band alignments for a single perovskite-based heterostructure. Here, we systematically investigate the electronic and optical properties of all-inorganic perovskite vdWHs CsSnBr₃/WS_2(1-x)Se_2x based on density functional theory (DFT) calculation. The calculated results show that the transitions of band alignment from type-II to type-I and type-III to type-II are achieved by modulating the doping ratio of the Se atom in the WS_2(1-x)Se_2x monolayer for SnBr₂/WS_2(1-x)Se_2x and CsBr/WS_2(1-x)Se_2x heterostructures, respectively, in which the CsBr and SnBr₂ represent two different terminated surfaces of CsSnBr₃. The change of band alignments can be attributed to the conduction band minimum (CBM) transforming from the W 5d to Sn 5p orbital in SnBr₂/WS_2(1-x)Se_2x vdWHs, and the valence band maximum (VBM) and CBM change from an overlapped state to a separated one in CsBr/WS_2(1-x)Se_2x vdWHs. This work can provide a theoretical basis for the dynamic modulation of band alignments in perovskite-based vdWHs.

Strain-Modulated Photoelectric Responses from a Flexible α-In2Se3/3R MoS2 Heterojunction

Semiconducting piezoelectric α-In₂Se₃ and 3R MoS₂ have attracted tremendous attention due to their unique electronic properties. Artificial van der Waals (vdWs) heterostructures constructed with α-In₂Se₃ and 3R MoS₂ flakes have shown promising applications in optoelectronics and photocatalysis. Here, we present the first flexible α-In₂Se₃/3R MoS₂ vdWs p-n heterojunction devices for photodetection from the visible to near infrared region. These heterojunction devices exhibit an ultrahigh photoresponsivity of 2.9 × 10³ A W^-1 and a substantial specific detectivity of 6.2 × 10¹⁰ Jones under a compressive strain of - 0.26%. The photocurrent can be increased by 64% under a tensile strain of + 0.35%, due to the heterojunction energy band modulation by piezoelectric polarization charges at the heterojunction interface. This work demonstrates a feasible approach to enhancement of α-In₂Se₃/3R MoS₂ photoelectric response through an appropriate mechanical stimulus.

Thickness-induced band-gap engineering in lead-free double perovskite Cs2AgBiBr6 for highly efficient photocatalysis

In recent years, two-dimensional (2D) lead-free double perovskites have been attracting much attention because of their unique performance in photovoltaic solar cells and photocatalysis. Nonetheless, how thickness affects the photoelectric properties of lead-free double perovskite remains unclear. In this work, by means of density functional theory (DFT) with a spin orbit coupling (SOC) effect, we have investigated the electronic and optical properties systemically, including band structures, carrier mobility, optical absorption spectra, exciton-binding energies, band edges alignment and molecule adsorption performance of Cs2AgBiBr6 with different thicknesses. The calculated results revealed the thickness-induced band gap and optical performance for Cs2AgBiBr6. It shows a low band gap and outstanding optical absorption of visible and ultraviolet light. When the thickness is reduced to a monolayer, Cs2AgBiBr6 moves from an indirect band gap to a direct band gap. Moreover, the carrier mobility of Cs2AgBiBr6 is excellent and the exciton-binding energy increases with the decreased thickness. Importantly, an analysis of molecule adsorption and band edge alignment indicates that Cs2AgBiBr6 is prone to H2O adsorption and H2 desorption theoretically, which is conducive to the photocatalytic water splitting for hydrogen generation and other photovatalytic reactions. Our work suggests that Cs2AgBiBr6 is a potential candidate as a solar cell or a photocatalyst, and we provide theoretical explorations into reducing the layers of lead-free double perovskite materials to 2D atomic thickness for a better photocatalytic application, which can serve as guidelines for the design of excellent photocatalysts.

A two-dimensional h-BN/C2N heterostructure as a promising metal-free photocatalyst for overall water-splitting

The construction of a heterostructure (HS) is an effective strategy to modulate the desired properties of two-dimensional (2D) materials and to extend their applications. In this paper, based on the density functional theory, we predict a metal-free type-II HS formed by h-BN and C₂N single layers. The h-BN/C₂N HS possesses a smaller bandgap than individual h-BN and C₂N single layers, and it exhibits excellent visible light absorption. Importantly, its band edge positions satisfy the requirements for spontaneous water-splitting. With the assistance of the built-in electric field across the HS and the band offset, the photoinduced carriers can be readily spatially separated. Free energy calculations indicate the high catalytic activity for water oxidation and reduction reactions. The performance can be further enhanced by strain, which modulates the bandgap and the band edge positions of the HS. The band alignment may undergo a transition from type-I to type-II under strain, offering an effective switch for the reaction. The appropriate bandgap, suitable band edge positions, and effective carrier separation make the h-BN/C₂N HS a promising candidate for use as a photocatalyst in water-splitting.