Van Der Waals Heterostructures Research Articles

Vertical stacking approach for tailoring electronic and optical properties of ultrathin two-dimensional vdW heterostructures of P-MAB (M=Ti, Zr, Hf; A=S, B=Se)

The vertical staking in the form of hetersotructures, is an effective way to enhance the performance of corresponding isolated monolayers. Strong interactions between Janus monolayers (MAB) and Phosphorus (P) monolayers result in significant improvements in both physical and chemical properties compared to single layer configurations. Using first- principle (hybrid) calculations, we explore electronic structure and optical properties of P-MAB (M = Ti, Zr, Hf; A = S, B=Se) van der Waals heterostructures, for potential applications in electronic and optoelectronic devices. Ab initio molecular dynamic (AIMD) simulations confirm the thermal stability, while phonon spectra confirm the dynamical stabilities of these systems. Our analysis reveals that all configurations maintain semiconducting nature with an indirect bandgap. The type-II band alignment in the P-MAB heterostructures is demonstrated through calculations of the partial density of states (PDOS). Bader charge analysis and planar-averaged charge density differences indicate an electron transfer from the P monolayers to the MAB monolayer. Furthermore, our investigation of the dielectric function ϵ2(ω) shows that the lowest energy shift are primarily influenced by excitonic effects.

Epitaxy of GaSe Coupled to Graphene: From In Situ Band Engineering to Photon Sensing.

2D semiconductors can drive advances in quantum science and technologies. However, they should be free of any contamination; also, the crystallographic ordering and coupling of adjacent layers and their electronic properties should be well-controlled, tunable, and scalable. Here, these challenges are addressed by a new approach, which combines molecular beam epitaxy and in situ band engineering in ultra-high vacuum of semiconducting gallium selenide (GaSe) on graphene. In situ studies by electron diffraction, scanning probe microscopy, and angle-resolved photoelectron spectroscopy reveal that atomically-thin layers of GaSe align in the layer plane with the underlying lattice of graphene. The GaSe/graphene heterostructure, referred to as 2semgraphene, features a centrosymmetric (group symmetry D3d) polymorph of GaSe, a charge dipole at the GaSe/graphene interface, and a band structure tunable by the layer thickness. The newly-developed, scalable 2semgraphene is used in optical sensors that exploit the photoactive GaSe layer and the built-in potential at its interface with the graphene channel. This proof of concept has the potential for further advances and device architectures that exploit 2semgraphene as a functional building block.

Theoretical insights of 2D Fe3GeTe2/In2Se3 multiferroic vdW heterostructure based gas sensors for high-performance NO detecting

The impact of the harmful gas NO on the atmospheric environment cannot be ignored, so it is necessary to develop efficient gas sensors to detect and absorb NO at room temperature. In this work, we systematically explored the electron transport properties and gas sensing properties of Fe3GeTe2, In2Se3 monolayers and its multiferroic van der Waals (vdW) heterostructures of Fe3GeTe2/In2Se3 for detecting NO by using density functional theory and nonequilibrium Green's function methods. The calculated adsorption energies, electron localisation functions, band structures and density of states indicate that its a chemisorption for the adsorption of NO on Fe3GeTe2 and a physisorption on In2Se3, while the specific adsorption style is highly dependent on the stacking and molecular adsorption sites of different Fe3GeTe2/In2Se3 structures. In addition, the pristine Fe3GeTe2 monolayer and Fe3GeTe2/In2Se3 based nanodevices realized a significant anisotropy for electron transport along the zigzag and armchair directions, and their anisotropic maximum current ratios in the zigzag to armchair directions were 1.90 and 1.89. Meanwhile, the NO sensitivity ratio of Fe3GeTe2 and Fe3GeTe2/In2Se3 based gas sensors can be up to 79 % and 107 %, respectively. This highlights a notably superior gas-sensitive performance of the Fe3GeTe2/In2Se3 heterostructure compared to the Fe3GeTe2 monolayer gas devices. These findings provide valuable references for the application of multiferroic heterostructures in the field of gas sensors.

GeTe/MoTe2 Van der Waals Heterostructures: Enabling Ultralow Voltage Memristors for Nonvolatile Memory and Neuromorphic Computing Applications.

Advanced electronic semiconducting Van der Waals heterostructures (HSs) are promising candidates for exploring next-generation nanoelectronics owing to their exceptional electronic properties, which present the possibility of extending their functionalities to diverse potential applications. In this study, GeTe/MoTe2 HS are explored for nonvolatile memory and neuromorphic-computing applications. Sputter-deposited Ag/GeTe/MoTe2/Pt HS cross-point devices are fabricated, and they demonstrate memristor behavior at ultralow switching voltages (VSET: 0.15V and VRESET: -0.14V) with very low energy consumption (≈30 nJ), high memory window, long retention time (104 s), and excellent endurance (105 cycles). Resistive switching is achieved by adjusting the interface between the Ag top electrode and the heterojunction switching layer. Cross-sectional transmission electron microscope images and conductive atomic force microscopy analysis confirm the presence of a conducting filament in the heterojunction switching layer. Further, emulating various synaptic functions of a biological synapse reveals that GeTe/MoTe2 HS can be utilized for energy-efficient neuromorphic-computing applications. A multilayer perceptron is implemented using the synaptic weights of the Ag/GeTe/MoTe2/Pt HS device, revealing high pattern accuracy (81.3%). These results indicate that HS devices can be considered a potential solution for high-density memory and artificial intelligence applications.

Construction of VO2@V2C MXene heterostructure toward superior-rate rechargeable aqueous zinc batteries

V2C MXene delivers excellent metallic properties and robust mechanical stability. However, its low capacity, easy volume expansion during charge/discharge and relatively short cycle life limits its further development. It is an effective strategy to build Van der Waals heterostructures that can shield the monolithic materials from defects and integrate the advantages of the monolithic materials. Herein, via a one-step hydrothermal method, a VO2@V2C heterostructure is successfully constructed by the controllable oxidation of V2C, which allows for shorter ion diffusion distances, faster ion transport and enhancement of hydrophilic properties of the material. The VO2@V2C heterostructures have incredible reversibility (440.9 mAh/g at 0.2 A/g), remarkable cycling stability (142.7 mAh/g at 1.0 A/g after 900 cycles) and superior-rate capacity (255.3 mAh/g at 6.0 A/g). Density functional theory (DFT) calculations revealed that VO2@V2C possesses excellent adsorption capacity for zinc ions and outstanding electrical conductivity. The heterogeneous interface between the two materials is conducive to the increase of the embedding sites of zinc ions, which promotes the homogeneous distribution and rapid diffusion of zinc ions in the materials, and consequently enhances the capacity and cycling reliability of the electrodes. It provides a new idea for the selection of cathode electrodes for aqueous zinc ion batteries (ZIBs) in present study.

Deep subwavelength topological edge state in a hyperbolic medium.

Topological photonics offers the opportunity to control light propagation in a way that is robust from fabrication disorders and imperfections. However, experimental demonstrations have remained on the order of the vacuum wavelength. Theoretical proposals have shown topological edge states that can propagate robustly while embracing deep subwavelength confinement that defies diffraction limits. Here we show the experimental proof of these deep subwavelength topological edge states by implementing periodic modulation of hyperbolic phonon polaritons within a van der Waals heterostructure composed of isotopically pure hexagonal boron nitride flakes on patterned gold films. The topological edge state is confined in a subdiffraction volume of 0.021 µm3, which is four orders of magnitude smaller than the free-space excitation wavelength volume used to probe the system, while maintaining the resonance quality factor above 100. This finding can be directly extended to and hybridized with other van der Waals materials to broadened operational frequency ranges, streamline integration of diverse polaritonic materials, and compatibility with electronic and excitonic systems.

Exploring novel CrSSe-N2CO2 (N = Ti, Zr, Hf) van der Waals heterostructures for multifunctional optoelectronic and thermoelectric applications

Due to their tunable bandgaps and excellent exciton activity, van der Waals heterostructures are ideal for optical applications and provide design flexibility for thermoelectric applications. A comprehensive investigation of the structural, electronic, optical, and thermoelectric properties of novel CrSSe-N2CO2 (N = Ti, Zr, Hf) van der Waals (vdW) heterostructures was carried out in the present work. The generalized gradient approximations (GGA) were employed within the framework of density functional theory. The optimized lattice constants revealed a small lattice discrepancy of approximately 1 % which is consistent with previously available experimental and theoretical data. Six distinct stacking patterns were considered to control monolayer orientation and the atomic positions were optimized to determine the most thermally stable configurations at 300 K using ab initio molecular dynamics (AIMD) which confirms that these vdW heterostructures can be synthesized experimentally. The electronic properties, particularly the band structures, revealed direct and indirect band gaps between 0.1 and 1.2 eV. The current study serves as a foundation for developing these vdW heterostructures specifically CrSSe-Hf2CO2, which was found dynamically, and thermally stable with desired properties for future technological applications.

Ultrathin natural biotite crystals as a dielectric layer for van der Waals heterostructure applications

Biotite, an iron-rich mineral belonging to the trioctahedral mica group, is a naturally abundant layered material (LM) exhibiting attractive electronic properties for application in nanodevices. Biotite stands out as a non-degradable LM under ambient conditions, featuring high-quality basal cleavage-a significant advantage for van der Waals heterostructure (vdWH) applications. In this work, we present the micro-mechanical exfoliation of biotite down to monolayers (1Ls), yielding ultrathin flakes with large areas and atomically flat surfaces. To identify and characterize the mineral, we conducted a multi-elemental analysis of biotite using energy-dispersive spectroscopy mapping. Additionally, synchrotron x-ray fluorescence and infrared nano-spectroscopy were employed to probe its iron content and vibrational signature in few-layer form, respectively, with sensitivity to the layer number. We have also observed good morphological and structural stability in time (up to 12 months) and no important changes in their physical properties after thermal annealing processes in ultrathin biotite flakes. Conductive atomic force microscopy evaluated its electrical capacity, revealing an electrical breakdown strength of approximately 1 V nm-1. Finally, we explore the use of biotite as a substrate and encapsulating LM in vdWH applications. We have performed optical and magneto-optical measurements at low temperatures. We find that ultrathin biotite flakes work as a good substrate for 1L-MoSe2, comparable to hexagonal boron nitride flakes, but it induces a small change of the 1L-MoSe2g-factor values, most likely due to natural impurities on its crystal structure. Furthermore, our results show that biotite flakes are useful systems to protect sensitive LMs such as black phosphorus from degradation for up to 60 days in ambient air. Our study introduces biotite as a promising, cost-effective LM for the advancement of future ultrathin nanotechnologies.

Large area, ultrafast, suppressed dark current and high-performance PtSe2/MoS2 van der Waals heterostructure based visible to NIR broadband photodetector

Group-10 transitional metal dichalcogenide, in particular Platinum selenide (PtSe2), have attracted substantial attention owing to its fascinating properties such as high carrier mobility, narrow bandgap, and high stability in contrast to other low bandgap 2D materials. However, in bare PtSe2, high dark current and lower sensitivity restrict to make high-performance broadband photodetector. Herein, a large area and stable PtSe2/MoS2 heterostructure based photodetector is fabricated which exhibits suppressed dark current and high-performance with broad 400-1200 nm detection, covering visible to near-infrared region (NIR). The PtSe2/MoS2 heterostructure device exhibits ∼103 order reduction in the dark current, high detectivity, and better stable response as compared to its bare PtSe2 device. Moreover, under NIR (900 nm) illumination, the PtSe2/MoS2 device demonstrates high responsivity of 17.9 AW−1 and high specific detectivity of 9.8 × 1012 Jones at amoderate bias of −5V. It exhibits ultra-fast response with rise/ fall time of 103 μs/117 μs corroborating its high performance. To understand the working of PtSe2/MoS2 photodetector, a detailed carrier transport mechanism is investigated based on the interface. This work provides a facile strategy to fabricate large area, fast response and high-performance broadband photodetector to build future optoelectronic devices.

Investigation of high-detectivity self-powered photodetectors of Rubrene/Bi2Se3 hybrid Van der Waals heterojunction

This paper investigates the Vander Waals heterostructure of organic rubrene and topological insulator Bi2Se3 with an atomically abrupt interface with exciting applications in electronic and optoelectronic devices. This organic/inorganic heterostructure (OIH) was prepared using a simple two-step physical vapor deposition process; based on this hybrid structure, a self-powered photodetector was then constructed. The Dirac surface state at the heterointerface enhances the splitting and transferring of photo-generated carriers, resulting in improved device characteristics. Under 1064 nm, the prepared photodetectors demonstrated a maximum responsivity (6.37 A/W), a high detectivity (3.42 × 1010 Jones), and ultrafast photoresponse speeds with rise time (1.17 µs) and decay time (1.59 µs), respectively. These promising results suggest that these PDs can be used in various photodetection applications and may lead to developing new devices.

Vibrational Fermi Resonance in Atomically Thin Black Phosphorus.

Fermi resonance is a phenomenon involving the hybridization of two coincidentally quasi-degenerate states that is observed in the vibrational or electronic spectra of molecules. Despite numerous examples in molecular systems, vibrational Fermi resonances in dispersive semiconducting systems remain largely unexplored due to the rarity of occurrence. Here we report a vibrational Fermi resonance in atomically thin black phosphorus. The Fermi resonance arises via anharmonic mixing of a fundamental Raman mode and a Davydov component of an infrared mode, leading to a doublet with mixed character. The extent of Fermi coupling can be modulated by the application of external biaxial strain. The consequences of Fermi hybridization are revealed by electronic resonance effects in the thickness-dependent and excitation-wavelength-dependent Raman spectrum, which is predicted by ab initio hybrid functional simulations including excitonic interactions. This work reveals new insight into electron-phonon coupling in black phosphorus and demonstrates a novel method for modulating Fermi resonances in 2D semiconductors.

Guidelines for accurate and efficient calculations of mobilities in two-dimensional semiconductors

Guidelines for accurate and efficient calculations of mobilities in two-dimensional semiconductors

High-throughput screening of 2D materials identifies p-type monolayer WS2 as potential ultra-high mobility semiconductor

Abstract2D semiconductors offer a promising pathway to replace silicon in next-generation electronics. Among their many advantages, 2D materials possess atomically-sharp surfaces and enable scaling the channel thickness down to the monolayer limit. However, these materials exhibit comparatively lower charge carrier mobility and higher contact resistance than 3D semiconductors, making it challenging to realize high-performance devices at scale. In this work, we search for high-mobility 2D materials by combining a high-throughput screening strategy with state-of-the-art calculations based on the ab initio Boltzmann transport equation. Our analysis singles out a known transition metal dichalcogenide, monolayer WS2, as the most promising 2D semiconductor, with the potential to reach ultra-high room-temperature hole mobilities in excess of 1300 cm2/Vs should Ohmic contacts and low defect densities be achieved. Our work also highlights the importance of performing full-blown ab initio transport calculations to achieve predictive accuracy, including spin–orbital couplings, quasiparticle corrections, dipole and quadrupole long-range electron–phonon interactions, as well as scattering by point defects and extended defects.

Ultra‐Fast Weak Light Detectors Based on van der Waals Stacked 2D Semiconductor/h‐BN Heterostructures

AbstractFast and sensitive detectors for weak light are crucial in various fields like real‐time monitoring, medical diagnosis, and astronomy. However, photodetectors using 2D materials face challenges in achieving both a low detection limit for weak light and fast response. Here, a vertical vdW graphene/indium selenide (InSe)/hexagonal boron nitride (h‐BN) heterostructure on a gold electrode is provided, demonstrating an ultra‐low detection limit of 47.4 nW cm−2 and an ultra‐fast response speed of 327 ns. Vertical photodetectors feature nanoscale channel length, thus reducing scattering and recombination of carriers. The high potential barrier of multilayer h‐BN effectively blocks carrier transport, resulting in ultra‐low dark current and minimal power dissipation. Additionally, enhanced light absorption in InSe by the metal‐insulator‐semiconductor structure improves the sensitivity to weak light detection. More importantly, the built‐in field at the graphene/InSe interface can promote the separation of photo‐generated carriers to improve the collection efficiency. These factors jointly reduce the detection limit and response time simultaneously for weak light. This vertical vdW heterostructure comprising a 2D semiconductor/h‐BN offers a solution to make the majority of 2D semiconductors applicable in ultra‐fast weak light detectors.

Encapsulation of linear carbon chains in single-walled carbon nanotubes: Towards 1D van der Waals heterostructures for high-efficiency excitonic solar cells

One-dimensional van der Waals heterostructures (1D vdW HTs) based on single-walled carbon nanotubes (SWCNTs) encapsulating linear carbon chains (LCCs) with tunable optoelectronic properties provide a fresh ground for efficient excitonic solar cells (XSCs) alternative to those based on 2D monolayer materials. As an effort to identify for the first time 1D vdW HTs for efficient XSCs application, we perform here a systematic theoretical investigation on the energetic and structural stability, optoelectronic and photovoltaic properties of 1D vdW HTs based on SWCNTs encapsulating 50 symmetrical LCCs of different lengths, and capped with different conjugated endgroups through comprehensive first-principles calculations. We showed the fundamental role of the length and the endgroups effects in modulating the optoelectronic properties of LCCs. Through exploration of the energetic stability of the 1D vdW HTs of LCCs@SWCNTs from the universal force field (UFF) including vdW interactions, we determined the optimal SWCNTs diameters for which the resulting 1D vdW HTs are stable. Then, the band alignment in the 50 stable 1D vdW HTs of LCCs@SWCNTs is examined. Intriguingly, stable LCCs@SWCNTs are identified as type-II 1D vdW HTs complying the prerequisite of XSCs with strong optical absorption in the UV–visible-NIR spectral regions, and power conversion efficiency (PCE) reaching well over 21 %. These findings unravel the enormous potential of the 1D vdW HTs in designing next-generation XSCs devices competitive with state-of-the-art solar cells.

Revealing Bipolar Photoresponse in Multiheterostructured WTe2-GaTe/ReSe2-WTe2 P-N Diode by Hybrid 2D Contact Engineering.

The van der Waals (vdW) heterostructures based on two-dimensional (2D) semiconducting materials have been thoroughly investigated with regard to practical applications. Recent studies on 2D materials have reignited attraction in the p-n junction, with promising potential for applications in both electronics and optoelectronics. 2D materials provide exceptional band structural diversity in p-n junction devices, which is rare in regular bulk semiconductors. In this article, we demonstrate a p-n diode based on multiheterostructure configuration, WTe2-GaTe-ReSe2-WTe2, where WTe2 acts as heterocontact with GaTe/ReSe2 junction. Our devices with heterocontacts of WTe2 showed excellent performance in electronic and optoelectronic characteristics as compared to contacts with basic metal electrodes. However, the highest rectification ratio was achieved up to ∼2.09 × 106 with the lowest ideality factor of ∼1.23. Moreover, the maximum change in photocurrent (Iph) is measured around 312 nA at Vds = 0.5 V. The device showed a high responsivity (R) of 4.7 × 104 m·AW-1, maximum external quantum efficiency (EQE) of 2.49 × 104 (%), and detectivity (D*) of 2.1 × 1011 Jones at wavelength λ = 220 nm. Further, we revealed the bipolar photoresponse mechanisms in WTe2-GaTe-ReSe2-WTe2 devices due to band alignment at the interface, which can be modified by applying different gate voltages. Hence, our promising results render heterocontact engineering of the GaTe-ReSe2 heterostructured diode as an excellent candidate for next-generation optoelectronic logic and neuromorphic computing.

Susceptible Detection of Organic Molecules Based on C3B/Graphene and C3N/Graphene van der Waals Heterojunction Gas Sensors.

Constructing van der Waals (vdW) heterostructures is a prospective approach that is essential for developing a new generation of functional two-dimensional (2D) materials and designing new conceptual nanodevices. Using density-functional theory combined with a nonequilibrium Green's function approach allows for the theoretical and systematic exploration of the electronic structure, transport properties, and sensitivity of organic small molecules adsorbed on 2D C3B/graphene (Gra) and C3N/Gra vdW heterojunctions. Calculations show the metallic properties of C3B/Gra and C3N/Gra after the formation of heterojunctions. Interestingly, the heterojunctions C3B/Gra (C3N/Gra) for the adsorption of small organic molecules (C2H2, C2H4, CH3OH, CH4, and HCHO) at the C3B (C3N) side are sensitive to the chemisorption of C2H2 and C2H4. Similarly, the Gra/C3B is chemisorbed for both C2H2 and C2H4 when adsorbed on Gra side, while it is only chemisorbed for C2H2 in Gra/C3N. Interestingly, all heterojunctions on different sides are physisorbed for CH3OH, CH4, and HCHO. Furthermore, the calculated I-V curves demonstrate that the devices based on the adsorption of C2H2 and C2H4 at each side of the heterojunction have remarkable anisotropy, in with the current being considerably greater in the zigzag direction than in the armchair direction. More specifically, with C2H2 adsorbed on the Gra side, the sensitivity along the armchair direction is up to 85.0% for Gra/C3B and close to 100% for Gra/C3N. This study reveals that C3B/Gra (C3N/Gra) heterojunctions with high selectivity, high anisotropy, and excellent sensitivity are highly prospective 2D materials for applications, which further contributes new insights into the development of future electronic nanodevices.

Two-dimensional tetragonal carbon nitride semiconductors with fascinating electronic/optical properties and low thermal conductivity

Abstract Exploring two-dimensional (2D) tetragonal carbon nitride materials is significant for unlocking new physical properties beyond those offered by traditional hexagonal lattices. In this work, we propose three theoretically stable 2D carbon nitride monolayers with tetragonal lattices, namely T-C3N, P-C3N, and PH-C5N4. Electronic structure calculations indicate that all three monolayers exhibit semiconducting characteristics, with T-C3N showing interesting flat band features. Additionally, these three carbon nitrides exhibit anisotropic and high carrier mobilities and excellent light absorption capabilities in the visible-light and near-infrared regions. Meanwhile, the calculated thermal conductivity (κp) of PH-C5N4 is 63.9 Wm−1K−1 at room temperature, significantly outperforming T-C3N (12.2 Wm−1K−1) and P-C3N (18.9 Wm−1K−1). Phonon scattering rates and Grüneisen parameters confirm the origin for the relatively high κp in PH-C5N4. Our study proposes three tetragonal carbon nitride structures with novel physical properties, which lays a theoretical foundation for the multifunctional applications of 2D carbon nitride materials.

Mechanical properties of freestanding few-layer graphene/boron nitride/polymer heterostacks investigated with local and non-local techniques.

van der Waals two-dimensional materials and heterostructures combined with polymer films continue to attract research attention to elucidate their functionality and potential applications. This study presents the fabrication and mechanical testing of 2D material heterostacks, consisting of few-layer boron nitride and graphene heterostructures synthesized via chemical vapor deposition, capped with a polymethyl methacrylate layer and suspended across ∼200 μm wide trenches using a combined wet-dry transfer method. The mechanical characterization of the heterostacks was performed using two independent approaches: (a) non-local testing with a custom-built tensile testing platform and (b) local load-displacement testing employing atomic force microscopy probes, complemented by finite element simulations. Both approaches provided new results, which are in good agreement with each other. Overall, our findings offer new insights into a combined load capacity in complex multi-material two-dimensional systems, and can contribute to advancing micro and nano-scale device designs and implementations.

Palladium based air-stable 2D penta-material’s heterostructures for water splitting applications

Heterostructures offer superior photocatalytic characteristics over their constituent counterparts due to their charge separation abilities. Here, we conduct a systematic study of a recently synthesized novel family of palladium-based pentagonal air-stable 2D monolayers, PdSe2, PdPSe, and PdPS, and their heterostructures using first-principles calculations for photocatalytic applications. Electronic band structure calculations reveal moderate bandgaps of 2.27 eV for PdSe2, 2.01 eV for PdPSe, and 2.25 eV for PdPS, indicating their suitability for photocatalytic water splitting. Moreover, to spatially separate and reduce the recombination possibility of photoinduced electron-hole pairs, we propose three van der Waals heterostructures: PdPSe/PdSe2, PdPS/PdSe2, PdPS/PdPS, with the corresponding bandgaps of 1.84 eV, 1.64 eV, and 1.65 eV, respectively. Based on work functions and the staggered band alignment of constituting monolayers, all three heterostructures are identified as type-II photocatalysts, which makes them notable photocatalyst. Additionally, band-edge potentials of PdPSe/PdSe2 and PdPS/PdSe2 confirm their suitability for overall water splitting via photocatalysis, whereas PdPS/PdPSe is suitable for oxygen evolution reactions only. The optical absorption spectra show the ability of each system to operate under a wide range of the spectrum, from visible light to high-energy (UV) regions. These characteristics make these systems valuable and attractive for photocatalytic applications.