Graphene Van Der Waals Heterostructures Research Articles

WS2-Graphene van der Waals Heterostructure as Promising Anode Material for Lithium-Ion Batteries: A First-Principles Approach.

In this work, we report the results of density functional theory (DFT) calculations on a van der Waals (VdW) heterostructure formed by vertically stacking single-layers of tungsten disulfide and graphene (WS2/graphene) for use as an anode material in lithium-ion batteries (LIBs). The electronic properties of the heterostructure reveal that the graphene layer improves the electronic conductivity of this hybrid system. Phonon calculations demonstrate that the WS2/graphene heterostructure is dynamically stable. Charge transfer from Li to the WS2/graphene heterostructure further enhances its metallic character. Moreover, the Li binding energy in this heterostructure is higher than that of the Li metal's cohesive energy, significantly reducing the possibility of Li-dendrite formation in this WS2/graphene electrode. Ab initio molecular dynamics (AIMD) simulations of the lithiated WS2/graphene heterostructure show the system's thermal stability. Additionally, we explore the effect of heteroatom doping (boron (B) and nitrogen (N)) on the graphene layer of the heterostructure and its impact on Li-adsorption ability. The results suggest that B-doping strengthens the Li-adsorption energy. Notably, the calculated open-circuit voltage (OCV) and Li-diffusion energy barrier further support the potential of this heterostructure as a promising anode material for LIBs.

Optically induced trion formation and its control in a MoS2/graphene van der Waals heterostructure.

Monolayer 2D transition metal dichalcogenides (TMDs) show high sensitivity to the local dielectric environment, leading to modulation of their optoelectronic properties. Here, we report on the formation of localized trions in a MoS2/few-layer graphene van der Waals heterostructure. We performed temperature-dependent photoluminescence and Raman studies down to 80 K, to understand the mechanism for localized charge excitation, which shows contrasting behaviour with MoS2/SiO2. We attribute trion formation to optically induced charge transfer from few-layer graphene to MoS2. Our theoretical analysis and simulations comparing the dielectric screening between MoS2/SiO2 and MoS2/few-layer graphene strongly suggest the dominance of excess charge carrier concentration over dielectric screening as the cause of trion formation. The concentration of charge carriers could be tuned actively with excitation power. Our findings provide an efficient approach for trion formation in MoS2 and explain the mechanism behind charge transfer in the MoS2/few-layer graphene heterostructure.

Electrochemical mechanisms of the reduction of N2 to NH3 catalyzed by TM anchored on Nvs-g-C3N4/graphene van der Waals heterostructure: Insights from a first-principles study

Nitrogen fixation is one of the most critical issues in chemical science and technology. However, developing efficient and stable electro(photo)catalysts remains a comprehensive challenge. Here, we constructed 21 single-atom heterostructures consisting of g-C3N4 with nitrogen vacancies and graphene (TM@Nvs-g-C3N4/graphene, TM=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Ag, Pd, Cd and Pt) and systematically investigated their stabilities and catalytic activities for the conversion from N2 to NH3. The calculations indicate that all heterostructures are thermodynamically stable. Among of them, 15 TM@Nvs-g-C3N4/graphene heterojunctions can activate N2, and effectively reduce N2 into NH3 by a distal or alternating mechanism. The Cr, Mo and Nb atoms anchored in Nvs-g-C3N4/graphene heterojunctions have higher activities and selectivities for nitrogen reduction reaction (NRR). In particular, Mo@Nvs-g-C3N4/graphene has the highest NRR performance. The Bader charge and PDOS analysis elucidate that the d-π* feedback bonds formed by 4d orbitals of Mo atom and π* orbitals of N2 plays a dominant role in the activation of N2. Altogether, the results obtained in this study could provide new avenues for the development of electrochemical catalyst for the reduction of N2 to NH3.

Ultrafast-programmable two-dimensional p–n homojunction for high-performance photovoltaics and optoelectronics

Two-dimensional (2D) materials are considered to be promising candidates for constructing revolutionary electronic devices. However, difficulties in controlling the polarity, concentration, and spatial distribution of charge carriers in 2D materials make the construction of 2D p–n junctions rather challenging. Here, we report the successful construction of ultrafast-programmable 2D p–n homojunctions with a semi-floating-gate configuration based on a vertically stacked molybdenum disulfide (MoS2)/hexagonal boron nitride/multilayer graphene van der Waals heterostructure. By partially electrostatically doping the MoS2 channel under different control-gate voltage pulses, three types of 2D homojunctions, including p–n, n+–n, and n–n, can be constructed. The 2D p–n homojunction can be programmed at an ultrafast speed of within 160 ns and exhibits a large rectification ratio of ∼104. Based on a modified Shockley equation, an ideality factor of ∼2.05 is extracted, indicating that the recombination process dominated the transport mechanism. The MoS2 2D p–n homojunction shows a maximum electrical power conversion efficiency of up to 2.66% under a weak light power of 0.61 nW and a high photovoltage responsivity of 5.72 × 109 V W−1. These results indicate that the ultrafast-programmable 2D p–n homojunction has great potential for use in high-performance photovoltaics and optoelectronics.

Electrochemical Mechanisms of the Reduction of N2 to Nh3 Catalyzed by Tm Anchored on Nvs-G-C3n4/Graphene Van Der Waals Heterostructure: Insights from a First-Principles Study

Electrochemical Mechanisms of the Reduction of N2 to Nh3 Catalyzed by Tm Anchored on Nvs-G-C3n4/Graphene Van Der Waals Heterostructure: Insights from a First-Principles Study

Highly Tunable Carrier Tunneling in Vertical Graphene-WS2-Graphene van der Waals Heterostructures.

Owing to the fascinating properties, the emergence of two-dimensional (2D) materials brings various important applications of electronic and optoelectronic devices from field-effect transistors (FETs) to photodetectors. As a zero-band-gap material, graphene has excellent electric conductivity and ultrahigh carrier mobility, while the ON/OFF ratio of the graphene FET is severely low. Semiconducting 2D transition metal chalcogenides (TMDCs) exhibit an appropriate band gap, realizing FETs with high ON/OFF ratio and compensating for the disadvantages of graphene transistors. However, a Schottky barrier often forms at the interface between the TMDC and metallic contact, which limits the on-state current of the devices. Here, we lift the two limits of the 2D-FET by demonstrating highly tunable field-effect tunneling transistors based on vertical graphene-WS2-graphene van der Waals heterostructures. Our devices show a low off-state current below 1 pA and a high ON/OFF ratio exceeding 106 at room temperature. Moreover, the carrier transport polarity of the device can be effectively tuned from n-type under small bias voltage to bipolar under large bias by controlling the crossover from a direct tunneling region to the Fowler-Nordheim tunneling region. Further, we find that the effective barrier height can be controlled by an external gate voltage. The temperature dependence of carrier transport demonstrates that both tunneling and thermionic emission contribute to the operation current at elevated temperature, which significantly enhances the on-state current of the tunneling transistors.

A van der Waals heterostructure based on nickel telluride and graphene with spontaneous high-frequency photoresponse

Terahertz detectors have potential applications in various fields including security inspection, biomedicine, and noninvasive quality inspection due to their ability to detect terahertz radiation. However, traditional detection materials have reached their bottlenecks due to difficulties in the breakthrough of fundamental principles for terahertz light. In this work, a terahertz detector based on a NiTe2–graphene van der Waals heterostructure has been developed to inhibit the dark current and thermal-agitation noise at room temperature. The hetero-integration of NiTe2 and graphene exhibits enhanced photon-absorption ability and its downconversion into a direct current. The experimental results show that the peak photoresponsivity of our photodetector is 1.31 A W−1 at 0.28 THz, and the corresponding noise equivalent power is 17.56 pW Hz−1/2, which rivals commercially thermal-based photodetectors. Our device has already shown capabilities of large-area imaging, fast speed, and high signal-to-noise ratio, which can be rendered as an important step for exploring topological semimetal optoelectronics.

Quantum-Well Bound States in Graphene Heterostructure Interfaces.

We present experimental evidence of electronic and optical interlayer resonances in graphene van der Waals heterostructure interfaces. Using the spectroscopic mode of a low-energy electron microscope (LEEM), we characterized these interlayer resonant states up to 10eV above the vacuum level. Compared with nontwisted, AB-stacked bilayer graphene (AB BLG), an ≈0.2 Å increase was found in the interlayer spacing of 30° twisted bilayer graphene (30°-tBLG). In addition, we used Raman spectroscopy to probe the inelastic light-matter interactions. A unique type of Fano resonance was found around the D and G modes of the graphene lattice vibrations. This anomalous, robust Fano resonance is a direct result of quantum confinement and the interplay between discrete phonon states and the excitonic continuum.

PH sensors based on amino-terminated carbon nanomembrane and single-layer graphene van der Waals heterostructures

The ability of graphene to transduce an adsorption event of ions into a detectable electrical signal has sparked a lot of interest for its use in sensors. However, a low concentration of the chemically active sites for binding analytes on the graphene surface has significantly prevented its applications so far. Here, we report on implementation of the van der Waals heterostructure based on a monolayer graphene and an ∼1-nm-thick molecular carbon nanomembrane (CNM) in a solution-gated field-effect transistor (FET) for pH sensing. The nondestructive functionalization of a graphene FET with the amino-terminated CNM (NH2-CNM) enables the induction of chemically active groups in the vicinity of the graphene sheet, maintaining its charge carrier transport properties. We applied complementary characterization techniques, including Raman spectroscopy, x-ray photoelectron spectroscopy, and optical and atomic force microscopy as well as field-effect and electrical impedance measurements to characterize the engineered NH2-CNM/graphene devices. We demonstrate their high pH resolution with a minimum detectable pH change of ∼0.01 at pH 2 and ∼0.04 at pH 12, with a response time in the range of seconds, and we apply an electrical double-layer model to rationalize the experimentally observed performance theoretically. The developed device concept enables the engineering of microscale pH sensors for applications in biological and environmental sciences.

Computational Prediction for Stable Structures of Graphene Van Der Waals Heterostructures Composed of Group-III-V Compounds

Two-dimensional (2D) materials with honeycomb structure such as graphene have attracted much attention due to their potential applications in nanoelectronics devices. [1] Furthermore, 2D van der Waals (vdW) heterostructures can be formed by stacking different types of 2D layered materials through the weak interlayer vdW interaction, and have attracted interest due to unique functionalities which are impossible in homogeneous bulk materials. [2,3] For instance, various graphene and hexagonal boron nitride (h-BN) related vdW heterostructures have been demonstrated and their physical properties have been discussed. On the other hand, a new class of 2D materials composed of traditional group III–V materials have been recently proposed. Lucking et al. predicted that the double-layer honeycomb (DLHC) structure where pair of single layer honeycomb structure forms interlayer bonds can be realized in various group III-V, II–VI, and I–VII materials, and some of these 2D materials exhibit exotic topological properties. [4] It has also been reported that most of group III–V and II–VI thin films are stabilized by forming the DLHC structure when the thickness deceases toward the 2D limit whereas a structure with alternating octagonal and square rings called a haeckelite structure is favorable beyond 3–15 monolayers depending on the constituent elements. [5] These findings suggest the realization of a new class of vdW heterostructures consisting of conventional 2D materials (such as graphene, h-BN, and transition-metal dichalcogenides) and DLHC structure. In this study, we explore new stable structures of vdW heterostructures composed of group III-V compounds on the basis of density functional calculations taking account of vdW interaction.The calculations for superlattices consisting of MoS2/AlAs heterostructure reveal that covalent Al-S bonds are formed between Al atoms of AlAs with zinc blende structure and S atoms of MoS2, which stabilizes zinc blende phase over the DLHC structure. This indicates that the combination of transition-metal dichalcogenides and group III-V compounds such as MoS2/AlAs hardly forms vdW heterostructures. On the other hand, we find that vdW heterostructures can be formed for the superlattices consisting of graphene and group III-V compounds (AlAs, AlSb, GaAs, GaSb, InP, InAs, InSb) when its thickness is two monolayers. Furthermore, the vdW heterostructure with DLHC structure in group III-V compounds is more stable than that with conventional zinc blende structure. Therefore, a stable structure whose atomic configurations are different from those of bulk phase is newly found for the combination of graphene and DLHC structure. The calculated biding energies are ranging from 0.22 to 0.30 eV/unit cell, indicating that this vdW heterostructures can be stabilized even at room temperature. The calculations of phonon dispersion also reveal that no imaginary frequencies are present for graphene/InAs with DLHC structure, suggesting its dynamical stability. This is due to nearly coherent in-plane lattice matching (misfit of ~0.1 %) between InAs with DLHC structure and graphene, as demonstrated in InAs nanowire growth by vdW epitaxy. [6] These calculated results suggest that diverse combinations and exotic electronic properties could be discovered in the vdW heterostructures consisting of graphene and group III-V compounds. Acknowledgements: This work was supported in part by the Grant-in-Aid for Scientific Research Grant (JP20K05324, JP19K05268, and JP16H06418) from the JSPS, and KIOXIA Corporation (former Toshiba Memory Corporation).

Theoretical study of SPP properties based on black phosphorus–graphene van der Waals heterostructure

We theoretically investigate the surface plasmon polariton (SPP) properties based on black phosphorus–graphene van der Waals heterostructures that can be tuned by both the incident light angle of black phosphorus and Fermi level of graphene. We elucidate the angle-dependent SPP properties for the hybrid structure from the visible regime to mid-infrared regime. The results show that the largest SPP propagation distance is obtained in the zigzag direction of black phosphorus except when the wavelength is tuned from 380 nm to 480 nm in the visible regime and 1 µm–1.2 µm in the near infrared regime. We also reveal the quasi-linear and quasi-square linear relationship between the SPP propagation distance and Fermi level of graphene for different regimes. Our work would be of great value to construct and optimize the angle-dependent and tunable SPP devices based on the hybridization of different 2D materials.

Molybdenum disulfide–graphene van der Waals heterostructures as stable and sensitive electrochemical sensing platforms

Atomic layers are sought after for molecular sensing due to their available high surface interaction area, and different types of monolayers are attempted for sensing in the recent past. However, their chemical stability towards these molecules is questioned in recent times and alternate methods need to be developed to circumvent such issues, while maintaining high sensitivity. Here, the van der Waals (vdWs) stacks of molybdenum disulfide (MoS2) and graphene are shown for their stable electrochemical sensing towards ascorbic acid (AA) and dopamine (DA)—two important biomolecules. AA is known to chemically react with MoS2 leading to unstable sensing platform, while here the graphene coverage is shown to protect the MoS2 even from low-energy plasma exposure while keeping the same high sensitivity. Upon proving the graphene-based protection of the sensor, such a sensing platform is shown for its applicability in DA sensing, where it is found to give a linear response in a wide range of concentrations (2.5 to 600 µmol·L−1) and even selective sensing in the presence of AA. Such a stack is found to be not merely giving protection to the beneath MoS2 layer but also the inter-layer charge transfer due to work function differences being beneficial in bringing fast and high sensitivity to the next-generation sensors and point-of-care devices.

Design of MoS2/Graphene van der Waals Heterostructure as Highly Efficient and Stable Electrocatalyst for Hydrogen Evolution in Acidic and Alkaline Media

The thermodynamically stable phase of molybdenum disulfide (MoS2) is evaluated as a promising and durable nonprecious-metal electrocatalyst toward the hydrogen evolution reaction (HER); however, its actual catalytic activity is restricted by an inert basal plane, low electronic conductivity, low density, and using efficiency of edged atoms. Moreover, 2D/2D van der Waals (vdws) heterostructures (HSs) with face-to-face contact can construct a highly coupled interface and are demonstrated to have immense potential for catalytic applications. In the present work, a 2D/2D hetero-layered architecture of an electrocatalyst, based on the alternate arrangement of ultrasmall monolayer MoS2 nanosheets (approximately 5-10 nm) and ultrathin graphene (G) sheets, is prepared by a facilely chemical process, which is named as MoS2/G HS. The unique structural characteristic of MoS2/G HS is in favor of accommodating more active sites as the centers of ad/desorption hydrogen and transferring and separating the charges at a coupled interface to improve the electronic conductivity and durability. The density functional theory calculation results further confirm that the alternately arranged G layers and MoS2 monolayers, as well as the expanded interplanar distance of 1.104 nm for MoS2/G HS, can exhibit a superior HER performance in both 0.5 M H2SO4 and 1.0 M KOH.

Adsorption behaviour of Si anchored on g-C3N4/graphene van der Waals heterostructure for selective sensing of toxic gases: Insights from a first-principles study

Monitoring the levels of toxic gas molecules could efficiently ensure the safety of human health and the environment. Herein, the adsorption behaviour, geometries, energies and electronic properties of H2S, CO, NO2, NH3 and CO2 gas molecules on the Si-doped g-C3N4/graphene surface were systematically investigated using density functional theory calculations. Based on the study of adsorption and desorption behaviour, we indicate that Si-doped g-C3N4/graphene was a suitable sensing material for H2S, CO and CO2, while being a disposable gas sensor for the detection of NH3 and NO2 due to their greater adsorption energy. The electronic properties of Si-doped g-C3N4/graphene heterostructure showed strong hybridisation between the adsorbate orbitals and interacting surfaces, as well as the remarkable presence of NH3 and NO2 orbitals around the Fermi level. The Si-doped g-C3N4/graphene were potentially good H2S, CO and CO2 sensors because of the short recovery time, shift in electrical conductivity, apparent charge transfer and reasonable adsorption energies. To sum up, the results obtained in this study could provide details about the process of gas sensing mechanism.

Direct Observation of Charge Injection of Graphene in the Graphene/WSe2 Heterostructure by Optical-Pump Terahertz-Probe Spectroscopy.

Charge transfer across the interface and interlayer coupling in the graphene van der Waals heterostructure, which is constructed by graphene and semiconducting transition metal dichalcogenides (TMDCs), is critical for their electronic and optoelectronic applications. Photo-induced charge injection from TMDC to graphene has been studied in several heterostructure photodetectors. However, the response time significantly varies among different reports, ranging from microseconds to milliseconds. In this work, using a graphene/WSe2 heterostructure as an example, we directly observe the carrier density change in graphene by time-resolved optical-pump terahertz (THz)-probe spectroscopy and show the ultrafast picosecond photoresponse of graphene. In the absence of photoexcitation, THz time-domain spectroscopic measurements show that WSe2 can transfer holes to graphene and pull down the Fermi level of graphene. After excitation by the ultrafast laser pulse, the transient THz response shows a rapid (∼0.35 ps) increase in the graphene conductivity mainly due to the hole injection from WSe2 into graphene. Unlike previous reports on band bend as the guidance mechanism for charge transfer, our results show that the relevant mechanism is the band offset across the atomically sharp interface.

Concepts of infrared and terahertz photodetectors based on vertical graphene van der Waals and HgTe-CdHgTe heterostructures

We review recently proposed concepts of infrared and terahertz photodetectors based on graphene van der Waals heterostructures and HgTe-CdHgTe quantum well heterostructures and demonstrate their potential.

Spontaneous doping on high quality talc-graphene-hBN van der Waals heterostructures

Steady doping, added to its remarkable electronic properties, would make graphene a valuable commodity in the solar cell market, as energy power conversion could be substantially increased. Here we report a graphene van der Waals heterostructure which is able to spontaneously dope graphene (p-type) up to n ~ 2.2 × 1013 cm−2 while providing excellent charge mobility ( ~ 25 000 cm2 V−1 s−1). Such properties are achieved via deposition of graphene on atomically flat layered talc, a natural and abundant dielectric crystal. Raman investigation shows a preferential charge accumulation on graphene-talc van der Waals heterostructures, which are investigated through the electronic properties of talc/graphene/hBN heterostructure devices. These heterostructures preserve graphene’s good electronic quality, verified by the observation of quantum Hall effect at low magnetic fields (B = 0.4 T) at T = 4.2 K. In order to investigate the physical mechanisms behind graphene-on-talc p-type doping, we performed first-principles calculations of their interface structural and electronic properties. In addition to potentially improving solar cell efficiency, graphene doping via van der Waals stacking is also a promising route towards controlling the band gap opening in bilayer graphene, promoting a steady n or p type doping in graphene and, eventually, providing a new path to access superconducting states in graphene, predicted to exist only at very high doping.

Tunable transmission of quantum Hall edge channels with full degeneracy lifting in split-gated graphene devices

Charge carriers in the quantum Hall regime propagate via one-dimensional conducting channels that form along the edges of a two-dimensional electron gas. Controlling their transmission through a gate-tunable constriction, also called quantum point contact, is fundamental for many coherent transport experiments. However, in graphene, tailoring a constriction with electrostatic gates remains challenging due to the formation of p–n junctions below gate electrodes along which electron and hole edge channels co-propagate and mix, short circuiting the constriction. Here we show that this electron–hole mixing is drastically reduced in high-mobility graphene van der Waals heterostructures thanks to the full degeneracy lifting of the Landau levels, enabling quantum point contact operation with full channel pinch-off. We demonstrate gate-tunable selective transmission of integer and fractional quantum Hall edge channels through the quantum point contact. This gate control of edge channels opens the door to quantum Hall interferometry and electron quantum optics experiments in the integer and fractional quantum Hall regimes of graphene.

Infrared photodetectors based on graphene van der Waals heterostructures

We propose and evaluate the graphene layer (GL) infrared photodetectors (GLIPs) based on the van der Waals (vdW) heterostructures with the radiation absorbing GLs. The operation of the GLIPs is associated with the electron photoexcitation from the GL valence band to the continuum states above the inter-GL barriers (either via tunneling or direct transitions to the continuum states). Using the developed device model, we calculate the photodetector characteristics as functions of the GL-vdW heterostructure parameters. We show that due to a relatively large efficiency of the electron photoexcitation and low capture efficiency of the electrons propagating over the barriers in the inter-GL layers, GLIPs should exhibit the elevated photoelectric gain and detector responsivity as well as relatively high detectivity. The possibility of high-speed operation, high conductivity, transparency of the GLIP contact layers, and the sensitivity to normally incident IR radiation provides additional potential advantages in comparison with other IR photodetectors. In particular, the proposed GLIPs can compete with unitravelling-carrier photodetectors.

High broad-band photoresponsivity of mechanically formed InSe-graphene van der Waals heterostructures.

High broad-band photoresponsivity of mechanically formed InSe-graphene van der Waals heterostructures is achieved by exploiting the broad-band transparency of graphene, the direct bandgap of InSe, and the favorable band line up of InSe with graphene. The photoresponsivity exceeds that for other van der Waals heterostructures and the spectral response extends from the near-infrared to the visible spectrum.