Electronic and Optical Properties of van der Waals Heterostructures Based on Two-Dimensional Perovskite (PEA)2PbI4 and Black Phosphorus.
ABSTRACT: Combining two-dimensional (2D) perovskites with other 2D materials to form a van der Waals (vdW) heterostructure has emerged as an intriguing way of designing electronic and optoelectronic devices. The structural, electronic, and optical properties of the 2D (PEA)2PbI4/black phosphorus (BP) [PEA:(C4H9NH3)+] vdW heterostructure have been investigated using first-principles calculations. We found that the (PEA)2PbI4/BP heterostructure shows a high stability at room temperature. It is demonstrated that the (PEA)2PbI4/BP heterostructure exhibits a type-I band arrangement with high carrier mobility. Moreover, the band gap and band offset of (PEA)2PbI4/BP can be effectively modulated by an external electric field, and a transition from semiconductor to metal is observed. The band edges of (PEA)2PbI4 and BP in the (PEA)2PbI4/BP heterostructure, which show significant changes with the external electric field, provide further support. Furthermore, the BP layers can enhance the light absorption of the (PEA)2PbI4/BP heterostructures. Our results indicate that the 2D perovskite and BP vdW heterostructures are competitive candidates for the application of low-dimensional photovoltaic and optoelectronic devices.
Project description:van der Waals (vdW) heterostructures based on two-dimensional (2D) semiconducting materials have been extensively studied for functional applications, and most of the reported devices work with sole mechanism. The emerging metallic 2D materials provide us new options for building functional vdW heterostructures via rational band engineering design. Here, we investigate the vdW semiconductor/metal heterostructure built with 2D semiconducting InSe and metallic 1T-phase NbTe<sub>2</sub>, whose electron affinity <i>χ</i><sub>InSe</sub> and work function <i>Φ</i><sub>NbTe<sub>2</sub></sub> almost exactly align. Electrical characterization verifies exceptional diode-like rectification ratio of >10<sup>3</sup> for the InSe/NbTe<sub>2</sub> heterostructure device. Further photocurrent mappings reveal the switchable photoresponse mechanisms of this heterostructure or, in other words, the alternative roles that metallic NbTe<sub>2</sub> plays. Specifically, this heterostructure device works in a photovoltaic manner under reverse bias, whereas it turns to phototransistor with InSe channel and NbTe<sub>2</sub> electrode under high forward bias. The switchable photoresponse mechanisms originate from the band alignment at the interface, where the band bending could be readily adjusted by the bias voltage. In addition, a conceptual optoelectronic logic gate is proposed based on the exclusive working mechanisms. Finally, the photodetection performance of this heterostructure is represented by an ultrahigh responsivity of ∼84 A/W to 532 nm laser. Our results demonstrate the valuable application of 2D metals in functional devices, as well as the potential of implementing photovoltaic device and phototransistor with single vdW heterostructure.
Project description:2D vertical van der Waals (vdW) heterostructures with atomically sharp interfaces have attracted tremendous interest in 2D photonic and optoelectronic applications. Band alignment engineering in 2D heterostructures provides a perfect platform for tailoring interfacial charge transfer behaviors, from which desired optical and optoelectronic features can be realized. Here, by developing a two-step chemical vapor deposition strategy, direct vapor growth of monolayer PbI<sub>2</sub> on monolayer transition metal dichalcogenides (TMDCs) (WS<sub>2</sub>, WSe<sub>2</sub>, or alloying WS<sub>2(1-</sub> <i><sub>x</sub></i> <sub>)</sub>Se<sub>2</sub> <i><sub>x</sub></i> ), forming bilayer vertical heterostructures, is demonstrated. Based on the calculated electron band structures, the interfacial band alignments of the obtained heterostructures can be gradually tuned from type-I (PbI<sub>2</sub>/WS<sub>2</sub>) to type-II (PbI<sub>2</sub>/WSe<sub>2</sub>). Steady-state photoluminescence (PL) and time-resolved PL measurements reveal that the PL emissions from the bottom TMDC layers can be modulated from apparently enhanced (for WS<sub>2</sub>) to greatly quenched (for WSe<sub>2</sub>) compared to their monolayer counterparts, which can be attributed to the band alignment-induced distinct interfacial charge transfer behaviors. The band alignment nature of the heterostructures is further demonstrated by the PL excitation spectroscopy and interlayer exciton investigation. The realization of 2D vertical heterostructures with tunable band alignments will provide a new material platform for designing and constructing multifunctional optoelectronic devices.
Project description:After the discovery of graphene, a lot of research has been conducted on two-dimensional (2D) materials. In order to increase the performance of 2D materials and expand their applications, two different layered materials are usually combined by van der Waals (vdW) interactions to form a heterostructure. In this work, based on first-principles calculation, some charming properties of the heterostructure constructed by Hf<sub>2</sub>CO<sub>2</sub>, AlN and GaN are addressed. The results show that Hf<sub>2</sub>CO<sub>2</sub>/AlN and Hf<sub>2</sub>CO<sub>2</sub>/GaN vdW heterostructures can keep their original band structure shape and have strong thermal stability at 300 K. In addition, the Hf<sub>2</sub>CO<sub>2</sub>/MN heterostructure has I-type band alignment structure, which can be used as a promising light-emitting device material. The charge transfer between the Hf<sub>2</sub>CO<sub>2</sub> and AlN (or GaN) monolayers is 0.1513 (or 0.0414) |<i>e</i>|. The potential of Hf<sub>2</sub>CO<sub>2</sub>/AlN and Hf<sub>2</sub>CO<sub>2</sub>/GaN vdW heterostructures decreases by 6.445 eV and 3.752 eV, respectively, across the interface. Furthermore, both Hf<sub>2</sub>CO<sub>2</sub>/AlN and Hf<sub>2</sub>CO<sub>2</sub>/GaN heterostructures have remarkable optical absorption capacity, which further shows the application prospect of the Hf<sub>2</sub>CO<sub>2</sub>/MN heterostructure. The study of this work provides theoretical guidance for the design of heterostructures for use as photocatalytic and photovoltaic devices.
Project description:The generation and manipulation of spin polarization at room temperature are essential for 2D van der Waals (vdW) materials-based spin-photonic and spintronic applications. However, most of the high degree polarization is achieved at cryogenic temperatures, where the spin-valley polarization lifetime is increased. Here, we report on room temperature high-spin polarization in 2D layers by reducing its carrier lifetime via the construction of vdW heterostructures. A near unity degree of polarization is observed in PbI<sub>2</sub> layers with the formation of type-I and type-II band aligned vdW heterostructures with monolayer WS<sub>2</sub> and WSe<sub>2</sub>. We demonstrate that the spin polarization is related to the carrier lifetime and can be manipulated by the layer thickness, temperature, and excitation wavelength. We further elucidate the carrier dynamics and measure the polarization lifetime in these heterostructures. Our work provides a promising approach to achieve room temperature high-spin polarizations, which contribute to spin-photonics applications.
Project description:Designing the electronic structures of the van der Waals (vdW) heterostructures to obtain high-efficiency solar cells showed a fascinating prospect. In this work, we screened the potential of vdW heterostructures for solar cell application by combining the group III-VI MX<sub>A</sub> (M = Al, Ga, In and X<sub>A</sub> = S, Se, Te) and elementary group VI X<sub>B</sub> (X<sub>B</sub> = Se, Te) monolayers based on first-principle calculations. The results highlight that InSe/Te vdW heterostructure presents type-II electronic band structure feature with a band gap of 0.88 eV, where tellurene and InSe monolayer are as absorber and window layer, respectively. Interestingly, tellurene has a 1.14 eV direct band gap to produce the photoexcited electron easily. Furthermore, InSe/Te vdW heterostructure shows remarkably light absorption capacities and distinguished maximum power conversion efficiency (PCE) up to 13.39%. Our present study will inspire researchers to design vdW heterostructures for solar cell application in a purposeful way.
Project description:Vertically stacked, layered van der Waals (vdW) heterostructures offer the possibility to design materials, within a range of chemistries and structures, to possess tailored properties. Inspired by the naturally occurring mineral merelaniite, this paper studies a vdW heterostructure composed of a MoS<sub>2</sub> monolayer and a PbS bilayer, using density functional theory. A commensurate 2D heterostructure film and the corresponding 3D periodic bulk structure are compared. The results find such a heterostructure to be stable and possess p-type semiconducting characteristics. Due to the heterostructure's weak interlayer bonding, its carrier mobility is essentially governed by the constituent layers; the hole mobility is governed by the PbS bilayer, whereas the electron mobility is governed by the MoS<sub>2</sub> monolayer. Furthermore, we estimate the hole mobility to be relatively high (~10<sup>6</sup> cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup>), which can be useful for ultra-fast devices at the nanoscale.
Project description:Neutral and charged excitons (trions) in atomically thin materials offer important capabilities for photonics, from ultrafast photodetectors to highly efficient light-emitting diodes and lasers. Recent studies of van der Waals (vdW) heterostructures comprised of dissimilar monolayer materials have uncovered a wealth of optical phenomena that are predominantly governed by interlayer interactions. Here, we examine the optical properties in NbSe<sub>2</sub>-MoSe<sub>2</sub> vdW heterostructures, which provide an important model system to study metal-semiconductor interfaces, a common element in optoelectronics. Through low-temperature photoluminescence (PL) microscopy, we discover a sharp emission feature, L1, that is localized at the NbSe<sub>2</sub>-capped regions of MoSe<sub>2</sub>. L1 is observed at energies below the commonly studied MoSe<sub>2</sub> excitons and trions and exhibits temperature- and power-dependent PL consistent with exciton localization in a confining potential. This PL feature is robust, observed in a variety of samples fabricated with different stacking geometries and cleaning procedures. Using first-principles calculations, we reveal that the confinement potential required for exciton localization naturally arises from the in-plane band bending due to the changes in the electron affinity between pristine MoSe<sub>2</sub> and NbSe<sub>2</sub>-MoSe<sub>2</sub> heterostructure. We discuss the implications of our studies for atomically thin optoelectronics devices with atomically sharp interfaces and tunable electronic structures.
Project description:Van der Waals heterostructures composed of two-dimensional (2D) transition metal dichalcogenides (TMD) materials have stimulated tremendous research interest in various device applications, especially in energy-efficient future-generation electronics. Such ultra-thin stacks as tunnel junction theoretically present unprecedented possibilities of tunable relative band alignment and pristine interfaces, which enable significant performance enhancement for steep-slope tunneling transistors. In this work, the optimal 2D-2D heterostructure for tunneling transistors is presented and elaborately engineered, taking into consideration both electric properties and material stability. The key challenges, including band alignment and metal-to-2D semiconductor contact resistances, are optimized separately for integration. By using a new dry transfer technique for the vertical stack, the selected WS<sub>2</sub>/SnS<sub>2</sub> heterostructure-based tunneling transistor is fabricated for the first time, and exhibits superior performance with comparable on-state current and steeper subthreshold slope than conventional FET, as well as on-off current ratio over 10<sup>6</sup> which is among the highest value of 2D-2D tunneling transistors. A visible negative differential resistance feature is also observed. This work shows the great potential of 2D layered semiconductors for new heterostructure devices and can guide possible development of energy-efficient future-generation electronics.
Project description:Great success in 2D van der Waals (vdW) heterostructures based photodetectors is obtained owing to the unique electronic and optoelectronic properties of 2D materials. Performance of photodetectors based 2D vdW heterojunctions at atomic scale is more sensitive to the nanointerface of the heterojunction than conventional bulk heterojunction. Here, a nanoengineered heterostructure for the first-time demonstration of a nanointerface using an inserted graphene layer between black phosphorus (BP) and InSe which inhibits interlayer recombination and greatly improves photodetection performances is presented. In addition, a transition of the transport characteristics of the device is induced by graphene, from diffusion motion of minority carriers to drift motion of majority carriers. These two reasons together with an internal photoemission effect make the BP/G/InSe-based photodetector have ultrahigh specific detectivity at room temperature. The results demonstrate that high-performance vdW heterostructure photodetectors can be achieved through simple structural manipulation of the heterojunction interface on nanoscale.
Project description:Mid-infrared (MIR) light-emitting devices play a key role in optical communications, thermal imaging, and material analysis applications. Two-dimensional (2D) materials offer a promising direction for next-generation MIR devices owing to their exotic optical properties, as well as the ultimate thickness limit. More importantly, van der Waals heterostructures-combining the best of various 2D materials at an artificial atomic level-provide many new possibilities for constructing MIR light-emitting devices of large tuneability and high integration. Here, we introduce a simple but novel van der Waals heterostructure for MIR light-emission applications built from thin-film BP and transition metal dichalcogenides (TMDCs), in which BP acts as an MIR light-emission layer. For BP-WSe<sub>2</sub> heterostructures, an enhancement of ~200% in the photoluminescence intensities in the MIR region is observed, demonstrating highly efficient energy transfer in this heterostructure with type-I band alignment. For BP-MoS<sub>2</sub> heterostructures, a room temperature MIR light-emitting diode (LED) is enabled through the formation of a vertical PN heterojunction at the interface. Our work reveals that the BP-TMDC heterostructure with efficient light emission in the MIR range, either optically or electrically activated, provides a promising platform for infrared light property studies and applications.