Search Results (64)

Van der Waals (vdW) heterostructures, typically composed of two-dimensional (2D) atomic layers, have attracted significant attention over the past few decades. Their performance is closely dependent on their composition and interlayer interactions. In this study, we constructed four types of 2D hexagonal BP monolayer (h-BP)/borophosphene vdW heterostructures with different stacking orders: (i) B-B stacking, (ii) P-P stacking, (iii) moire-I, and (iv) moire-II. Their structural stability and their electronic and optical properties were explored by using first-principles calculations. The results show that h-BP/borophosphene heterostructures can maintain their configurations with good structural stability and minimal lattice mismatch. All vdW heterostructures exhibit semiconducting characteristics, and their band gaps are highly dependent on interlayer stacking orders. Due to the regular atomic arrangement and enhanced interlayer dipole interactions, the B-B stacking bilayer opens a relatively large band gap of 0.157 eV, while the moire-II bilayer exhibits a very small band gap of 0.045 eV because of its irregular atom arrangements. By calculating the complex dielectric function, optical absorption spectra of B-B and P-P stacking bilayers were discussed. This study suggests that h-BP/borophosphene heterostructures have desirable optical properties, broadening the potential applications of the constituent monolayers. Full article

Electrical transport in 2D materials exhibits unique behaviors due to reduced dimensionality, broken symmetries, and quantum confinement. It serves as both a sensitive probe for the emergence of coherent electronic phases and a tool to actively manipulate many-body correlated states. Exploring their interplay and interdependence is crucial but remains underexplored. This review integratively cross-examines the atomic and electronic structures and transport properties of van der Waals-layered crystals ZrTe₃, 2H-TaS₂, and Cr₂Si₂Te₆, providing a comprehensive understanding and uncovering new discoveries and insights. A common observation from these crystals is that modifying the atomic and electronic interface structures of 2D van der Waals interfaces using heteroatoms significantly influences the emergence and stability of coherent phases, as well as phase-sensitive transport responses. In ZrTe₃, substitution and intercalation with Se, Hf, Cu, or Ni at the 2D vdW interface alter phonon–electron coupling, valence states, and the quasi-1D interface Fermi band, affecting the onset of CDW and SC, manifested as resistance upturns and zero-resistance states. We conclude here that these phenomena originate from dopant-induced variations in the lattice spacing of the quasi-1D Te chains of the 2D vdW interface, and propose an unconventional superconducting mechanism driven by valence fluctuations at the van Hove singularity, arising from quasi-1D lattice vibrations. Short-range in-plane electronic heterostructures at the vdW interface of Cr₂Si₂Te₆ result in a narrowed band gap. The sharp increase in in-plane resistance is found to be linked to the emergence and development of out-of-plane ferromagnetism. The insertion of 2D magnetic layers such as Mn, Fe, and Co into the vdW gap of 2H-TaS₂ induces anisotropic magnetism and associated transport responses to magnetic transitions. Overall, 2D vdW interface modification offers control over collective electronic behavior, transport properties, and their interplays, advancing fundamental science and nanoelectronic devices. Full article

The interaction between terahertz (THz) photons and phonons of materials is crucial for the development of THz photonics. In this work, typical two-dimensional (2D) van der Waals (vdW) transition metal chalcogenide (TMD) layers and heterostructures are used in THz time-domain spectroscopy (TDS) measurements, low-wavenumber Raman spectroscopy measurements, calculation of 2D materials’ phonon spectra, and theoretical analysis of thermal responses. The TDS results reveal strong absorption of THz photons in the frequency range of 2.5–10 THz. The low-wavenumber Raman spectra show the phonon vibration characteristics and are used to establish phonon energy bands. We also set up a computational simulation model for thermal responses. The temperature increases and distributions in the individual layers and their heterostructures are calculated, showing that THz photon absorption results in significant increases in temperature and differences in the heterostructures. These give rise to interesting photothermal effects, including the Seebeck effect, resulting in voltages across the heterostructures. These findings provide valuable guidance for the potential optoelectronic application of the 2D vdW heterostructures. Full article

This paper reports possible performance improvements of split-off band infrared detectors by using novel quantum materials. The report starts by describing the development of split-off band infrared detectors based on heterostructures with extended photoresponsivity beyond the energy band gap. The design modification demonstrated a new phenomenon of extending the threshold wavelength beyond the standard wavelength threshold (λ_t) determined by the energy gap (Δ) and the wavelength equation λ_t = 1.24/Δ with the dark current still governed by the original energy gap. However, to overcome the operating temperature challenges in AlGaAs/GaAs-based devices, the perspective of van der Waals quantum materials (vdW-QM)-based IR sensors is discussed regarding the aspects of heterostructure fabrication methods, theoretical modeling, and strategies that could help to overcome these issues. Through these discussions, the review paper aims to inspire upcoming innovations in developing novel IR photodetectors capable of operating within the atmospheric window at room temperature. Full article

The advent of two-dimensional (2D) ferroelectrics offers a new paradigm for device miniaturization and multifunctionality. Recently, 2D α-In₂Se₃ and related III–VI compound ferroelectrics manifest room-temperature ferroelectricity and exhibit reversible spontaneous polarization even at the monolayer limit. Here, we employ first-principles calculations to investigate group-III selenide van der Waals (vdW) heterojunctions built up by 2D α-In₂Se₃ and α-Ga₂Se₃ ferroelectric (FE) semiconductors, including structural stability, electrostatic potential, interfacial charge transfer, and electronic band structures. When the FE polarization directions of α-In₂Se₃ and α-Ga₂Se₃ are parallel, both the α-In₂Se₃/α-Ga₂Se₃ P↑↑ (UU) and α-In₂Se₃/α-Ga₂Se₃ P↓↓ (NN) configurations possess strong built-in electric fields and hence induce electron–hole separation, resulting in carrier depletion at the α-In₂Se₃/α-Ga₂Se₃ heterointerfaces. Conversely, when they are antiparallel, the α-In₂Se₃/α-Ga₂Se₃ P↓↑ (NU) and α-In₂Se₃/α-Ga₂Se₃ P↑↓ (UN) configurations demonstrate the switchable electron and hole accumulation at the 2D ferroelectric interfaces, respectively. The nonvolatile characteristic of ferroelectric polarization presents an innovative approach to achieving tunable n-type and p-type conductive channels for ferroelectric field-effect transistors (FeFETs). In addition, in-plane biaxial strain modulation has successfully modulated the band alignments of the α-In₂Se₃/α-Ga₂Se₃ ferroelectric heterostructures, inducing a type III–II–III transition in UU and NN, and a type I–II–I transition in UN and NU, respectively. Our findings highlight the great potential of 2D group-III selenides and ferroelectric vdW heterostructures to harness nonvolatile spontaneous polarization for next-generation electronics, nonvolatile optoelectronic memories, sensors, and neuromorphic computing. Full article

ZnSb is widely recognized as a promising thermoelectric material in its bulk form, and a ZnSb bilayer was recently synthesized from the bulk. In this study, we designed a vertical van der Waals heterostructure consisting of a ZnSb bilayer and an h-BN monolayer to investigate its electronic, elastic, transport, and thermoelectric properties. Based on density functional theory, the results show that the formation of this heterostructure significantly enhances electron mobility and reduces the bandgap compared to the ZnSb bilayer, thereby increasing its power factor. These findings highlight the potential of the h-BN monolayer–supported ZnSb bilayer heterostructure in thermoelectric applications, where maximizing energy conversion efficiency is essential. Full article

The advent of two-dimensional (2D) materials and their capacity to form van der Waals (vdW) heterostructures has revolutionized numerous scientific fields, including electronics, optoelectronics, and energy storage. This paper presents a comprehensive investigation of bandgap engineering and band structure prediction in 2D vdW heterostructures utilizing density functional theory (DFT). By combining various 2D materials, such as graphene, hexagonal boron nitride (h-BN), transition metal dichalcogenides, and blue phosphorus, these heterostructures exhibit tailored properties that surpass those of individual components. Bandgap engineering represents an effective approach to addressing the limitations inherent in material properties, thereby providing enhanced functionalities for a range of applications, including transistors, photodetectors, and solar cells. Furthermore, this study discusses the current limitations and challenges associated with bandgap engineering in 2D heterostructures and highlights future prospects aimed at unlocking their full potential for advanced technological applications. Full article

Polar van der Waals (vdW) crystals, composed of atomic layers held together by vdW forces, can host phonon polaritons—quasiparticles arising from the interaction between photons in free-space light and lattice vibrations in polar materials. These crystals offer advantages such as easy fabrication, low Ohmic loss, and optical confinement. Recently, hexagonal boron nitride (hBN), known for having hyperbolicity in the mid-infrared range, has been used to explore multiple modes with high optical confinement. This opens possibilities for practical polaritonic nanodevices with subdiffractional resolution. However, polariton waves still face exposure to the surrounding environment, leading to significant energy losses. In this work, we propose a simple approach to inducing a hyperbolic phonon polariton (HPhP) waveguide in hBN by incorporating a low dielectric medium, ZrS₂. The low dielectric medium serves a dual purpose—it acts as a pathway for polariton propagation, while inducing high optical confinement. We establish the criteria for the HPhP waveguide in vdW heterostructures with various thicknesses of ZrS₂ through scattering-type scanning near-field optical microscopy (s-SNOM) and by conducting numerical electromagnetic simulations. Our work presents a feasible and straightforward method for developing practical nanophotonic devices with low optical loss and high confinement, with potential applications such as energy transfer, nano-optical integrated circuits, light trapping, etc. Full article

Van der Waals (vdW) heterostructures are mainly fabricated by a classic dry transfer procedure, but the interface quality is often subject to the vdW gap, residual strains, and defect species. The realization of interface fusion and repair holds significant implications for the modulation of multiple photoelectric conversion processes. In this work, we propose a thermally mismatched strategy to trigger broad-band and high-speed photodetection performance based on a type-I heterostructure composed of black phosphorus (BP) and FePS₃ (FPS) nanoflakes. The BP acts as photothermal source to promote interface fusion when large optical power is adopted. The regulation of optical power enables the device from pyroelectric (PE) and/or alternating current photovoltaic (AC–PV) mode to a mixed photovoltaic (PV)/photothermoelectric (PTE)/PE mode. The fused heterostructure device presents an extended detection range (405~980 nm) for the FPS. The maximum responsivity and detectivity are 329.86 mA/W and 6.95 × 10¹⁰ Jones, respectively, and the corresponding external quantum efficiency (EQE) approaches ~100%. Thanks to these thermally-related photoelectric conversion mechanism, the response and decay time constants of device are as fast as 290 μs and 265 μs, respectively, superior to current all FPS-based photodetectors. The robust environmental durability also renders itself as a high-speed and broad-band imaging sensor. Full article

Constructing heterostructures from already synthesized two-dimensional materials is of significant importance. We performed a first-principles study to investigate the electronic properties and interfacial characteristics of Janus MoSH/WSi₂N₄ van der Waals heterostructure (vdWH) contacts. We demonstrate that the p-type Schottky formed by MoSH/WSi₂N₄ and MoHS/WSi₂N₄ has extremely low Schottky barrier heights (SBHs). Due to its excellent charge injection efficiency, Janus MoSH may be regarded as an effective metal contact for WSi₂N₄ semiconductors. Furthermore, the interfacial characteristics and electronic structure of Janus MoSH/WSi₂N₄ vdWHs can not only reduce/eliminate SBH, but also forms the transition from p-ShC to n-ShC type and from Schottky contact (ShC) to Ohmic contact (OhC) through the layer spacing and electric field. Our results can offer a fresh method for optoelectronic applications based on metal/semiconductor Janus MoSH/WSi₂N₄ vdW heterostructures, which have strong potential in optoelectronic applications. Full article

The assembly of van der Waals (vdW) heterostructures using 2D material transfer systems has revolutionized the field of materials science, enabling the development of novel electronic and optoelectronic devices and the probing of emergent phenomena. The innovative vertical stacking methods enabled by these 2D material transfer systems are central to constructing complex devices, which are often challenging to achieve with traditional bottom-up nanofabrication techniques. Over the past decade, vdW heterostructures have unlocked numerous applications leading to the development of advanced devices, such as transistors, photodetectors, solar cells, and sensors. However, achieving consistent performance remains challenging due to variations in transfer processes, contamination, and the handling of air-sensitive materials, among other factors. Several of these challenges can be addressed through careful design considerations of transfer systems and through innovative modifications. This mini-review critically examines the current state of transfer systems, focusing on their design, cost-effectiveness, and operational efficiency. Special emphasis is placed on low-cost systems and glovebox integration essential for handling air-sensitive materials. We highlight recent advancements in transfer systems, including the integration of cleanroom environments within gloveboxes and the advent of robotic automation. Finally, we discuss ongoing challenges and the necessity for further innovations to achieve reliable, cleaner, and scalable vdW technologies for future applications. Full article

Two-dimensional transition metal dichalcogenides (2D-TMDs) possess appropriate bandgaps and interact via van der Waals (vdW) forces between layers, effectively overcoming lattice compatibility challenges inherent in traditional heterojunctions. This property facilitates the creation of heterojunctions with customizable bandgap alignments. However, the prevailing method for creating heterojunctions with 2D-TMDs relies on the low-efficiency technique of mechanical exfoliation. Sb₂Te₃, recognized as a notable p-type semiconductor, emerges as a versatile component for constructing diverse vertical p–n heterostructures with 2D-TMDs. This study presents the successful large-scale deposition of 2D Sb₂Te₃ onto inert mica substrates, providing valuable insights into the integration of Sb₂Te₃ with 2D-TMDs to form heterostructures. Building upon this initial advancement, a precise epitaxial growth method for Sb₂Te₃ on pre-existing WS₂ surfaces on SiO₂/Si substrates is achieved through a two-step chemical vapor deposition process, resulting in the formation of Sb₂Te₃/WS₂ heterojunctions. Finally, the development of 2D Sb₂Te₃/WS₂ optoelectronic devices is accomplished, showing rapid response times, with a rise/decay time of 305 μs/503 μs, respectively. Full article

Van der Waals (vdW) heterostructures provide an effective strategy for exploring and expanding the potential applications of two-dimensional materials. In this study, we employ first-principles density functional theory (DFT) to investigate the geometric, electronic, and optical properties of MoGe₂N₄/AlN and MoSiGeN₄/AlN vdW heterostructures. The stable MoGe₂N₄/AlN heterostructure exhibits an indirect band gap semiconductor with a type-I band gap arrangement, making it suitable for optoelectronic devices. Conversely, the stable MoSiGeN₄/AlN heterostructure demonstrates various band gap arrangements depending on stacking modes, rendering it suitable for photocatalysis applications. Additionally, we analyze the effects of mechanical strain and vertical electric field on the electronic properties of these heterostructures. Our results indicate that both mechanical strain and vertical electric field can adjust the band gap. Notably, application of an electric field or mechanical strain leads to the transformation of the MoGe₂N₄/AlN heterostructure from a type-I to a type-II band alignment and from an indirect to a direct band transfer, while MoSiGeN₄/AlN can transition from a type-II to a type-I band alignment. Type-II band alignment is considered a feasible scheme for photocatalysis, photocells, and photovoltaics. The discovery of these characteristics suggests that MoGe₂N₄/AlN and MoSiGeN₄/AlN vdW heterostructures, despite their high lattice mismatch, hold promise as tunable optoelectronic materials with excellent performance in optoelectronic devices and photocatalysis. Full article

In this study, we explore the exciton dynamics in a WS₂/MoS₂ van der Waals (vdW) heterostructure under varying pressures by integrating a laser-confocal photoluminescence (PL) spectroscope and an atomic force microscope (AFM). For the WS₂/MoS₂ heterostructure, the exciton emission belonging to MoS₂ is too weak to be distinguished from the PL spectra. However, upon contact with a Si probe, the emission intensity of WS₂ excitons significantly decreases from 34,234 to 6560, thereby matching the intensity level of MoS₂. This alteration substantially facilitates the exploration of interlayer excitonic properties within the heterostructures using PL spectroscopy. Furthermore, the Si probe can apply out-of-plane localized pressure to the heterostructure. With increasing pressure, the emission intensity of the WS₂ trions decreases at a rate twice that of other excitons, and the exciton energy increases at a rate of 0.1 meV nN⁻¹. These results elucidate that the WS₂ trions are particularly sensitive to the out-of-plane pressure within a WS₂/MoS₂ vdW heterostructure. Full article

Two-dimensional (2D) vertical van der Waals heterostructures (vdWHs) show great potential across various applications. However, synthesizing large-scale structures poses challenges owing to the intricate growth parameters, forming unexpected hybrid film structures. Thus, precision in synthesis and thorough structural analysis are essential aspects. In this study, we successfully synthesized large-scale structured 2D transition metal dichalcogenides (TMDs) via chemical vapor deposition using metal oxide (WO₃ and MoO₃) thin films and a diluted H₂S precursor, individual MoS₂, WS₂ films and various MoS₂/WS₂ hybrid films (Type I: Mo_xW_1−xS₂ alloy; Type II: MoS₂/WS₂ vdWH; Type III: MoS₂ dots/WS₂). Structural analyses, including optical microscopy, Raman spectroscopy, transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy, and cross-sectional imaging revealed that the A_1g and E_2g modes of WS₂ and MoS₂ were sensitive to structural variations, enabling hybrid structure differentiation. Type II showed minimal changes in the MoS_2′s A_1g mode, while Types I and III exhibited a ~2.8 cm⁻¹ blue shift. Furthermore, the A_1g mode of WS₂ in Type I displayed a 1.4 cm⁻¹ red shift. These variations agreed with the TEM-observed microstructural features, demonstrating strain effects on the MoS₂–WS₂ interfaces. Our study provides insights into the structural features of diverse hybrid TMD materials, facilitating their differentiation through Raman spectroscopy. Full article