vdW Heterojunctions Research Articles

High-performance self-powered photodetectors based on the carbon nanomaterial/GaAs vdW heterojunctions

The predecessor of China Optics was China Optics and Applied Optics Digest, which was founded in 1985. At that time, it was the only retrieval journal in the field of optics in China. At the end of 2008, China Optics and Applied Optics Digest was renamed

2D Violet phosphorene with highly anisotropic mobility and its vdW heterojunction design for device applications.

Recently, the crystal structure of violet phosphorus and its monolayer violet phosphorene (VP) have been reconfirmed experimentally, and they were verified to be more thermally stable than their allotrope, black phosphorus. Here, we calculated the carrier mobility of monolayer VP using density functional theory. It is found that the carrier mobility is highly anisotropic and the hole mobility reaches 9.86 × 103 cm2 V-1 s-1 in the a-direction, endowing the potential application of VP in p-type semiconductor channel materials. Moreover, the Schottky barrier of the graphene/VP heterojunction turns into an ohmic contact when the electric field strength is >2 V nm-1. Therefore, VP and graphene/VP heterojunctions have potential prospects in electronic devices.

Two-Dimensional Semiconductor Heterojunctions for Optoelectronics and Electronics

Two-dimensional (2D) layered semiconductors are current research hotspots on account of their wide variety of applications in electronics and optoelectronics due to their particular ultrathin nature. In this review, the band alignment engineering in heterojunctions composed of 2D van der Waals (vdW) layered semiconductors and their device applications in optoelectronics are provided. Various approaches that induced adjustability of vdW heterojunctions are summarized, mainly including composition and thickness modulations, strain, and electric fields. Furthermore, their perspectives on future developments in optoelectronics and electronics devices based on the newly unique physical and chemical properties are outlined.

Direct observation of trapped charges at ReSe2 and graphene heterojunctions

The van der Waals (vdW) heterojunction often reveals unexpected characteristics distinct from conventional junctions. We investigate an emergent interface phenomenon between monolayer ReSe2 and graphene via combined studies of scanning tunneling microscopy (STM) and density functional theory (DFT). When probing monolayer ReSe2 on graphene at bias voltages within the ReSe2 band gap (in-gap bias; −1.2 V to 0.5 V), strikingly, observed topograph appears just like ReSe2 as it shows precisely the same hexagonal periodicity of ReSe2 lattice instead of the underlying graphene lattice. To answer this puzzle, we examine the nature of charge redistribution at the confined vdW gap between ReSe2 and graphene. DFT calculations indicate that the electron accumulation arises right below the ReSe2 layer, while the electron depletion occurs from the graphene layer. This leads to an asymmetrically polarized two-dimensional layer of confined electron density (termed as trapped charges) in the vdW gap, which agrees well with the in-gap STM topograph. We also find that the accumulation of the trapped charge is enhanced strongly at the edge of ReSe2.

Boosting Photocatalytic Activity Using Carbon Nitride Based 2D/2D van der Waals Heterojunctions

The surging demand for energy and staggering pollutants in the environment have geared the scientific community to explore sustainable pathways that are economically feasible and environmentally compelling. In this context, harnessing solar energy using semiconductor materials to generate charge pairs to drive photoredox reactions has been envisioned as a futuristic approach. Numerous inorganic crystals with promising nanoregime properties investigated in the past decade have yet to demonstrate practical application due to limited photon absorption and sluggish charge separation kinetics. Two-dimensional semiconductors with tunable optical and electronic properties and quasi-resistance-free lateral charge transfer mechanisms have shown great promise in photocatalysis. Polymeric graphitic carbon nitride (g-C3N4) is among the most promising candidates due to fine-tuned band edges and the feasibility of optimizing the optical properties via materials genomics. Constructing a two-dimensional (2D)/2D van der Waals (vdW) heterojunction by allies of 2D carbon nitride sheets and other 2D semiconductors has demonstrated enhanced charge separation with improved visible photon absorption, and the performance is not restricted by the lattice matching of constituting materials. With the advent of new 2D semiconductors over the recent past, the 2D/2D heterojunction assemblies are gaining momentum to design high performance photocatalysts for numerous applications. This review aims to highlight recent advancements and key understanding in carbon nitride based 2D/2D heterojunctions and their applications in photocatalysis, including small molecules activation, conversion, and degradations. We conclude with a forward-looking perspective discussing the key challenges and opportunity areas for future research.

Interface controlled band alignment type in Janus SnS2/SSnSe and SnS2/SeSnS van der Waals heterojunctions

Two-dimensional (2D) Janus structures have been employed significantly considering their asymmetrical layout and multiple applications in different areas. A novel van der Waals (vdW) heterojunction is constructed based on the SnS2 monolayer and Janus SnSSe monolayer in the current study. Due to different atoms in two sides of the SnSSe monolayer, two kinds of heterojunctions, including SnS2/SSnSe and SnS2/SeSnS, can be constructed. Both heterojunctions are indirect band gap semiconductors. The kind of interface atom can affect the electronic properties of heterojunctions, resulting in interface-controlled band alignment (BA) type. Since the conduction band minimum states of the SnS2/SSnSe system contain the contribution of both SSnSe and SnS2 layers, this system is not suitable for efficient electron-hole separation. The SnS2/SeSnS heterojunction constructs a standard type-II BA, in which the holes and electrons are placed within the layers of SeSnS and SnS2, respectively. The external electric field, in-plane strain, and the adsorption of nonmetal atoms are also considered to modulate the electronic properties and BA type of the heterojunctions. According to these findings, the interface atom is an important factor influencing the electronic properties of those Janus vdW heterojunctions.

Coexistence of Photoelectric Conversion and Storage in van der Waals Heterojunctions.

Van der Waals (vdW) heterojunctions, based on two-dimensional (2D) materials, have great potential for the development of ecofriendly and high-efficiency nanodevices, which shows valuable applications as photovoltaic cells, photodetectors, etc. However, the coexistence of photoelectric conversion and storage in a single device has not been achieved until now. Here, we demonstrate a simple strategy to construct a vdW p-n junction between a WSe_{2} layer and quasi-2D electron gas. After an optical illumination, the device stores the light-generated carriers for up to seven days, and then releases a very large photocurrent of 2.9mA with bias voltage applied in darkness; this is referred to as chargeable photoconductivity (CPC), which completely differs from any previously observed photoelectric phenomenon. In normal photoconductivity, the recombination of electron-hole pairs occurs at the end of their lifetime; in contrast, infinite-lifetime photocarriers can be generated and stored in CPC devices without recombination. The photoelectric conversion and storage are completely self-excited during the charging process. The ratio between currents in full- and empty-photocarrier states below the critical temperature reaches as high as 10^{9}, with an external quantum efficiency of 93.8% during optical charging. A theoretical model developed to explain the mechanism of this effect is in good agreement with the experimental data. This work paves a path toward the high-efficiency devices for photoelectric conversion and storage.

Multifunctional silicene/CeO2 heterojunctions: Desirable electronic material and promising water-splitting photocatalyst

The first-principles calculations demonstrate that covalently bonded (cb) heterojunction and van der Waals (vdW) heterojunction can coexist in silicene/CeO2 heterojunctions, due to the different stacking patterns. Especially, the cb heterojunction with band gap of 1.97 eV, forms a type-II heterojunction, exhibits good redox performance and has high-effective optical absorption spectra, thus it is a promising photocatalyst for overall water splitting. Besides, for the vdW heterojunction, the Dirac cone of silicene is well kept on CeO2 semiconducting substrate, with a considerable energy gap of 0.43 eV, which can be an ideal material in building silicene-based electronic device. These results may open a new gateway in both of nanoelectronic device and energy conversion for silicene/CeO2 nanocomposites.

Strain and electric field tunable electronic and optical properties in antimonene/C3N van der Waals heterostructure

Two-dimensional (2D) crystalline carbonitride C3N, as a very attractive material, has received extensive attention. By using the first-principles calculations, we systematically studied the electronic properties of antimonene/C3N van der Waals (vdW) heterojunction. The results show that the heterostructure has an inherent type-II band alignment with an indirect band gap. Moreover, the band gap of the C3N/antimonene heterojunction can be adjusted linearly within a certain range by applying vertical strain and electric field. The calculated absorption curve shows that the antimonene/C3N heterostructure can exhibit good visible-light absorption performance, and the optical properties of the C3N/antimonene heterojunction can be changed by applying biaxial and vertical strain, which can be expected to have potential applications in photovoltaic devices.

Hybrid van der Waals heterojunction based on two-dimensional materials

Since the discovery of graphene, two-dimensional (2D) layered materials have always been the focus of material research. The layers of 2D materials are covalent bonds, and the layers are weakly bonded to adjacent layers through van der Waals (vdW) interactions. Since any dangling-bond-free surface could be combined with another material through vdW forces, the concept can be extended. This can refer to the integration of 2D materials with any other non-2D materials through non-covalent interactions. The emerging mixed-dimensional (2D+nD, where n is 0, 1 or 3) heterostructure devices has been studied and represents a wider range of vdW heterostructures. New electronic devices and optoelectronic devices based on such heterojunctions have unique functions. Therefore, this article depicts the research progress of (2D+nD, where n is 0, 1 or 3) vdW heterojunctions based on 2D materials.

Tunable properties of ZnSe/graphene heterostructure as a promising candidate for photo/electro-catalyst applications

The graphene-based Van der Waals (vdW) heterostructures show great potential for the next generation of photoelectronic nanodevices. Here, we constructed the ZnSe/graphene vdW heterojunction and investigated its structural, electronic, optical, transportable, and catalytic properties by using density functional theory (DFT). The results show that ZnSe is a direct semiconductor with a bandgap of 3.24 eV. The electron mobility of ZnSe is seven times higher than that of the hole, which is beneficial for the photogenerated hole-electron separation. Besides, the n-type Schottky to Ohmic contact transformation can be realized by applying an external electric field of 0.2 V/Å, and the underlying mechanism is discussed. Finally, the potential ability of photo/electrocatalysis for graphene/ZnSe heterostructure and electrocatalysis for ZnSe monolayer is also explored. The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) can spontaneously activate in acid (PH = 1,2) and alkaline (PH > 9) media under light irradiation. The overpotential of oxygen reduction reaction reaches 0.42 eV, which indicates graphene/ZnSe heterostructure would be a potential electrocatalyst. For the HER process of ZnSe monolayer, the overpotential is significantly decreased to 0.23 eV by doping 3d transition metal atom. Consequently, we hope our work can provide useful guidance for designing practical ZnSe-based applications.

Multifunctional Optoelectronic Synapse Based on Ferroelectric Van der Waals Heterostructure for Emulating the Entire Human Visual System

AbstractThe development of optoelectronic synapses can provide an important breakthrough toward creating a sophisticated and adaptable artificial visual system analogous to that of humans. However, it remains a great challenge to implement the various functions of the biological visual neuromorphic system at the single device level. Intriguingly, 2D van der Waals (vdW) heterostructure may offer a platform to address the issue. Here, a novel multifunctional synaptic device based on ferroelectric α‐In2Se3/GaSe vdW heterojunction is proposed to emulate the entire biological visual system. Essential synaptic behaviors are observed in response to light and electrical stimuli; additionally, the retina‐like selectivity for light wavelengths and the achievement of Pavlov's dog experiment demonstrate the device's capacity for processing complex electrical and optical inputs. Beyond the optoelectronic synaptic behaviors, the device incorporates memory and logic functions analogous to those in the brain's visual cortex. The results of artificial neural network simulations show that the vdW heterojunction‐based device is completely capable of performing logic operations and recognizing images with a high degree of accuracy. The study indicates that versatile devices with a rationally designed construction have great potential for efficiently processing complex visual information and may simplify the design of artificial visual systems.

Defective g-C3N4/covalent organic framework van der Waals heterojunction toward highly efficient S-scheme CO2 photoreduction

In this work, a novel van der Waals (vdW) heterojunction composite combining g-C3N4 with nitrogen vacancies and Tp-Tta COF manifests effective interface contact area and excellent photocatalytic CO2 reduction performance. First-principles density functional theory calculation and experimental results suggest that the presence of nitrogen vacancies in g-C3N4 can widen the Fermi level gap between C3N4 (NH) and Tp-Tta COF, promoting the recombination of invalid photogenerated carriers through S-scheme pathway. Benefitted from the accelerated transfer of photogenerated charges at the vdW heterostructure interface, the deactivation of oxygen vacancies in C3N4 (NH)/COF is prevented and much higher photocatalytic activity and stability are obtained. The efficient electron transfer and the affinity of Tp-Tta for CO2 are beneficial to the enhanced CO selectivity. This work provides insights for the design of S-scheme heterojunction photocatalyst for CO2 reduction.

Two-dimensional g-C3N4/Ti2CO2 heterostructure as a direct Z-scheme photocatalyst for water splitting: A hybrid density functional theory investigation

By using first-principles calculations based on hybrid density functional theory, we investigate the electronic structures and photocatalytic properties of two-dimensional (2D) g-C 3 N 4 /Ti 2 CO 2 van der Waals (vdW) heterostructure. Results show that g-C 3 N 4 /Ti 2 CO 2 heterostructure has a staggered type-II band alignment, and a built-in electric field is formed at the interface due to significant charge transfer from g-C 3 N 4 to Ti 2 CO 2 . Interestingly, we observed spatially distinguished conduction band and valence band edges, and a direct Z-scheme photocatalytic mechanism can be established in the heterojunction, favorable for suppressed charge carrier recombination. We further demonstrate high catalytic activity of g-C 3 N 4 /Ti 2 CO 2 towards both hydrogen evolution and water oxidation reactions, and the heterojunction exhibits excellent light-harvesting capability with entire visible light absorption. These results not only provide fundamental insight into the photocatalyst with Z-scheme mechanism, but also shed light on future design and fabrication of 2D vdW heterojunctions as high-performance catalysts for technologically important chemical reactions. • Electronic properties of 2D g-C 3 N 4 /TiCO 2 vdW heterostructure are investigated by hybrid functional calculations. • It exhibits spatially distinguished conduction/valence band edges, and a built-in electric field is formed. • As a direct Z-scheme photocatalyt, g-C 3 N 4 /TiCO 2 has high catalytic activity towards hydrogen and oxygen evolution. • It demonstrates strong optical adsorption in the entire visible light region.

Fourfold Polarization‐Sensitive Photodetector Based on GaTe/MoS2 van der Waals Heterojunction

AbstractIntegrated polarization‐sensitive photodetectors fabricated by geometric anisotropic 2D materials have become attractive in recent years. In this work, the successful construction of self‐driven and polarization‐sensitive photodetectors based on GaTe/MoS2 p–n van der Waals (vdW) heterojunction is demonstrated by mechanical exfoliation and dry transfer methods. The fabricated GaTe/MoS2 vdW heterojunctions show ambipolar behavior, and the highest rectification ratio can reach 93.4. The highest responsivity under 532 nm illumination reaches 145 mA W−1 and the response time is less than 10 ms. Moreover, the photocurrent polarization of the fabricated GaTe/MoS2 photodetectors manifests in fourfold anisotropy with a high polarization ratio of 2.9, which can be ascribed to the highly anisotropic monoclinic structure of layered m‐GaTe. This finding thus offers more information and creates new opportunities about how to fabricate integrated polarization‐sensitive photodetectors.

Two-dimensional PtSe2/hBN vdW heterojunction as photoelectrocatalyst for the solar-driven oxygen evolution reaction: A first principles study

Designing and researching suitable photoelectrocatalyst for water splitting is crucial for the utilization of renewable and inexhaustible solar energy but remains a huge conundrum. Here, using first principles calculations, the two-dimensional PtSe2/hBN heterojunction was first conceived. However, the rapid recombination of electrons and holes will inhibit the visible light catalytic efficiency of the heterojunction. In order to realize effective spatial separation of electrons and holes of the heterojunction, we tuned the band gap by doping two carbon atoms on the hBN monolayer to obtain the type-Ⅱ PtSe2/hBNC heterojunction. Then through the comprehensive researches on structural, electronic and optical properties, it was found that the structure possesses excellent carrier mobility and exhibits a great oxygen evolution reaction performance with a low overpotential of 0.76 eV, which is an ideal photoelectrocatalyst and exhibits great light absorption performance. In this study, the possible mechanism of doped PtSe2/hBNC vdW heterojunction to promote photocatalysis and oxygen evolution ability was explained, which may pave the way for the practical design of more solar-driven high-quality water splitting photoelectrocatalysts.

Fabrication of 2D PdSe2/3D CdTe Mixed-Dimensional van der Waals Heterojunction for Broadband Infrared Detection.

Room-temperature infrared photodetectors are in great demand because of their vitally important applications in the military and civilian fields, which has inspired intensive studies in recent decades. Here, we present the fabrication of a large-size PdSe2/CdTe mixed-dimensional van der Waals (vdW) heterojunction for room-temperature infrared detection. Taking advantage of the wide light absorption of the multilayer PdSe2, high-quality vdW interface, and unique mixed-dimensional device geometry, the present device is capable of detecting an ultrawide light up to long-wave infrared (LWIR) of 10.6 μm at room temperature. In addition, our photodetector exhibits a good capability to follow short pulse infrared signals with a quick response rate of 70 ns, a large responsivity of 324.7 mA/W, and a reasonable specific detectivity of 3.3 × 1012 Jones. Significantly, the assembled photodetector is highly sensitive to a polarized infrared light signal with a decent polarization sensitivity of 4.4. More importantly, an outstanding LWIR imaging capability based on the PdSe2/CdTe vdW heterojunction at nonrefrigeration condition is demonstrated. Our work paves a novel route for the design of highly polarization-sensitive, room-temperature infrared photodetectors.

Tunable electronic and optical properties of new two-dimensional CuCl/GaSe van der Waals heterostructures with the potential for photocatalysis applications

The construction of a heterojunction photocatalyst can enhance the absorption of visible light and reduce the recombination of electrons and holes, which greatly improves the photocatalytic performance, which is also a current research hotspot in the energy field. In this paper, the first-principles calculation method is used to systematically study the electronic structure and photocatalytic mechanism of the new CuCl/GaSe van der Waals (vdW) heterojunction. The main research contents are summarized as follows: CuCl/GaSe vdW heterojunction is a direct band gap semiconductor material with a lattice mismatch rate of less than 1.5%, and a stable structure. The band gap of the heterojunction is 1.143 e V, which corresponds to a wider visible light absorption range. And it shows direct energy band alignment. The photo-generated holes are transferred from the GaSe layer to the CuCl layer, while the electrons are on the contrary, indicating that the electrons and holes are well separated in space. All these indicate that the CuCl/GaSe vdW heterojunction has good photocatalytic performance and is a new type of photocatalytic material with excellent performance.

Type-II vdW heterojunction SeGa2Te/SeIn2Se as a high-efficiency visible-light-driven water-splitting photocatalyst

Two-dimensional (2D) van der Waals (vdW) materials have been widely adopted as photocatalysts in water splitting. Based on the first-principles calculations within screened hybrid functional of Heyd-Scuseria-Ernzerhof (HSE), we find that Janus group-III chalcogenide based vdW heterojunction (SeGa2Te/SeIn2Se) is a promising visible-light-driven photocatalyst regardless on the stacking configuration. The SeGa2Te/SeIn2Se in AA stacking satisfy the requirement for the band gap under both pH = 0 and pH = 7 according to standard water-splitting mechanism. The notable intrinsic electric field (v-EF) in the AB stacking of SeGa2Te/SeIn2Se alter the band alignment and enhance the overpotentials of the photogenerated carriers to efficiently drive the redox reaction. In both stacking, the type-II band alignment, suitable direct band gaps, high optical absorbance and high power conversion efficiency promote the photocatalytic efficiency. Furthermore, outstanding photocatalytic ability and efficiency are maintained under both vertical and in-plane strains, guaranteeing the application under natural environment.

2D van der Waals Heterojunction of Organic and Inorganic Monolayers for High Responsivity Phototransistors

AbstractVan der Waals (vdW) heterostructures composing of organic molecules with inorganic 2D crystals open the door to fabricate various promising hybrid devices. Here, a fully ordered organic self‐assembled monolayer (SAM) to construct hybrid organic–inorganic vdW heterojunction phototransistors for highly sensitive light detection is used. The heterojunctions, formed by layering MoS2 monolayer crystals onto organic [12‐(benzo[b]benzo[4,5]thieno[2,3‐d]thiophen‐2‐yl)dodecyl)]phosphonic acid SAM, are characterized by Raman and photoluminescence spectroscopy as well as Kelvin probe force microscopy. Remarkably, this vdW heterojunction transistor exhibits a superior photoresponsivity of 475 A W−1 and enhanced external quantum efficiency of 1.45 × 105%, as well as an extremely low dark photocurrent in the pA range. This work demonstrates that hybridizing SAM with 2D materials can be a promising strategy for fabricating diversified optoelectronic devices with unique properties.