Graphene Heterostructures Research Articles

Disentangling Light‐Induced Charge Transfer, Heating, and Strain Effects in WS2/Graphene Heterostructures

AbstractHeterostructures formed by 2D transition metal dichalcogenides (TMDCs) and graphene are not only fundamentally important for the exploration of novel interface physics but also indispensable for various optoelectronic devices. Often multiple effects such as charge transfer (CT), heating, and strain are present simultaneously and their interplay significantly impacts the properties of such heterostructures. However, the identification and separation of these three individual effects are challenging and have not been demonstrated in 2D heterostructures. Here, through a concurrent analysis of the Raman modes and photoluminescence spectra from graphene and WS2 in temperature‐ and power‐dependent measurements, this work has succeeded in disentangling the effects of light‐induced CT, heating, and strain effects in WS2/graphene heterostructure on SiO2/Si substrate. Without disentangling these effects, the light‐induced carrier density in graphene (WS2) would have been underestimated (overestimated) by up to 62% (73%) in this situation. More importantly, a positive temperature coefficient of the G‐mode Raman shift is obtained without excluding the strain and CT effects, contrary to the negative coefficient due to the intrinsic thermal effect. This study demonstrates the importance of disentangling the individual effects and the approach paves the way for a comprehensive understanding of electrical, thermal, and mechanical properties of TMDC/graphene heterostrucutres.

UV light controlled optoelectronic memory based on WSe2 and hBN encapsulated graphene heterostructures

In recent times, 2D materials-based heterostructures are extensively studied for fabricating different nanodevices. Among these materials, transition metal dichalcogenides (TMDC), hexagonal boron nitride (hBN), and graphene (Gr) are widely used in these devices. Optoelectronic memory devices based on 2D materials are among the recently studied devices for structural flexibility and device size miniaturization. In this study, a non-volatile optoelectronic memory device has been fabricated using tungsten diselenide (WSe2), and hBN-encapsulated Gr-based heterostructures. This device can be easily controlled with UV light and an electric field. Two points were focused on in this study; first, the doping state was changed partially i.e., by illuminating a part of the WSe2 layer by UV light and a p-n homo-junction was created. This phenomenon occurs due to the unique structure of Wse2/hBN and the p-n junction could be created in one layer. Secondly, Gr has been used as a floating gate to increase the retention time as Gr is acting as a charge trapping layer. The retention time is further increased due to the thickness of hBN because the defect states keep increasing the charges together.

Electric field tunable layer polarization in graphene/boron-nitride twisted quadrilayer superlattices

The recently observed unconventional ferroelectricity in AB bilayer graphene sandwiched by hexagonal Boron Nitride (hBN) presents a new platform to manipulate correlated phases in multilayered van der Waals heterostructures. We present a low-energy continuum model for AB bilayer graphene encapsulated by the top and bottom layers of either hBN or graphene, with two independent twist angles. For the graphene/hBN heterostructures, we show that twist angle asymmetry leads to a layer polarization of the valence and conduction bands. We also show that an out-of-plane displacement field not only tunes the layer polarization but also flattens the low-energy bands. We extend the model to show that the electronic structures of quadrilayer graphene heterostructure consisting of AB bilayer graphene encapsulated by the top and bottom graphene layers can similarly be tuned by an external electric field.

Sensitivity Enhanced Plasmonic Biosensor Using Bi2Se3-Graphene Heterostructures: A Theoretical Analysis.

This study provided a theoretical insight for designing novel plasmonic biosensors using bismuth selenide (Bi2Se3)-Graphene heterostructures. It was a van der Waals (vdWs) stacked configuration composed of gold (Au) film, few quintuple layer (QL) Bi2Se3 and few-layered graphene. In particular, the proposed biosensor was created by Goos-Hänchen (GH) shift rather than phase, resulting in a more sensitive biosensing response. Under the excitation of 632.8 nm, significant sensitivity enhancement performance was obtained via varying the thickness of Bi2Se3-Graphene heterostructures. The best configuration was 32 nm Au film-2-QL Bi2Se3-3-layer graphene, generating the largest GH shift, as high as -1.0202 × 104 µm. Moreover, the highest detection sensitivity was determined to be 8.5017 × 106 µm/RIU, responding to a tiny refractive index (RI) change of 0.0012 RIU (RIU, refractive index unit). More importantly, our proposed biosensor has shown a theoretical feasibility of monitoring virus samples. For example, there was an efficient linear detection range for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2, 0~13.44 nanomole (nM)) and its Spike (S) glycoprotein (0~59.74 nM), respectively. It is expected that our proposed plasmonic biosensor has a potential application in performing sensitive detection of SARS-CoV-2.

Graphene and 2D Hexagonal Boron Nitride Heterostructure for Thermal Management in Actively Tunable Manner.

Thermal management is a critical task for highly integrated or high-power semiconductor devices. Low dimensional materials including graphene and single-layer hexagonal boron nitride (BN) are attractive candidates for this task because of their high thermal conductivity, semi-conductivity and other excellent physical properties. The similarities in crystal structure and chemistry between graphene and boron nitride provide the possibility of constructing graphene/BN heterostructures bearing unique functions. In this paper, we investigated the interfacial thermal transport properties of graphene/BN nanosheets via non-equilibrium molecular dynamics (NEMD) simulations. We observed a significant thermal rectification behavior of these graphene/BN nanosheets, and the rectification ratio increased with the system length increases up to 117%. This phenomenon is attributed to the mismatch of out-of-plane phonon vibration modes in two directions at the interface. In addition, we explored the underlying mechanism of the length dependence of the thermal transport properties. The results show promise for the thermal management of this two-dimensional heterostructure in an actively tunable manner.

Quantum Plasmonic Nonreciprocity in Parity-Violating Magnets.

The optical responses of metals are often dominated by plasmonic resonances, that is, the collective oscillations of interacting electron liquids. Here we unveil a new class of plasmons─quantum metric plasmons (QMPs)─that arise in a wide range of parity-violating magnetic metals. In these materials, a dipolar distribution of the quantum metric (a fundamental characteristic of Bloch wave functions) produces intrinsic nonreciprocal bulk plasmons. Strikingly, QMP nonreciprocity manifests even when the single-particle dispersion is symmetric: QMPs are sensitive to time-reversal and parity violations hidden in the Bloch wave function. In materials with asymmetric single-particle dispersions, quantum metric dipole induced nonreciprocity can continue to dominate at large frequencies. We anticipate that QMPs can be realized in a wide range of parity-violating magnets, including twisted bilayer graphene heterostructures, where quantum geometric quantities can achieve large values.

A Sensitive Photoelectrochemical Sensor for Levodopa Detection Using Benzothiadiazole-Based Conjugated Microporous Polymer-Coated Graphene Heterostructures.

Since the 1960s, levodopa (LDA) has been the standard drug for treating of Parkinson's disease. In this study, a novel benzothiadiazole-based conjugated microporous polymer-coated graphene heterostructure (CMP-rGO) was synthesized and used to construct a sensitive photoelectrochemical (PEC) sensor capable of detecting LDA. Under optimal experimental conditions, the intensity of the photocurrent produced by the sensor was linear from 0.005 to 40 μM, and the limit of detection of the sensor was 0.0027 μM. The sensor showed good repeatability, stability, and selectivity for LDA detection. Finally, the constructed sensor was used to detect LDA in levodopa tablets, human serum samples, and urine samples and satisfactory results were obtained. Therefore, the PEC sensor provides a novel platform for the detection of LDA in real samples and broadens the applications of conjugated microporous polymers in PEC sensing.

Correlated states in alternating twisted bilayer-monolayer-monolayer graphene heterostructure

Highly controlled electronic correlation in twisted graphene heterostructures has gained enormous research interests recently, encouraging exploration in a wide range of moiré superlattices beyond the celebrated twisted bilayer graphene. Here we characterize correlated states in an alternating twisted Bernal bilayer–monolayer–monolayer graphene of ∼ 1.74°, and find that both van Hove singularities and multiple correlated states are asymmetrically tuned by displacement fields. In particular, when one electron per moiré unit cell is occupied in the electron-side flat band, or the hole-side flat band (i.e., three holes per moiré unit cell), the correlated peaks are found to counterintuitively grow with heating and maximize around 20 K – a signature of Pomeranchuk effect. Our multilayer heterostructure opens more opportunities to engineer complicated systems for investigating correlated phenomena.

A first principle study of pressure‐induced properties of MoS2–graphene heterostructures

AbstractWe examine the capabilities of the ab‐initio calculations within density functional theory (DFT) on the 2D monolayer MoS2, graphene, and Gr‐MoS2 heterostructure. The stability, elastic properties, acoustics, electronic and optical properties of the three hexagonal MoS2, graphene, and Gr‐MoS2 heterostructure is presented. The energy bandgap and both Total and partial density of states are calculated within the PBE functional and are enhanced by the HSE06 functional. Gr‐MoS2 hold an indirect bandgap (0.294 ev) while MoS2 is a direct bandgap (1.83 eV); moreover, graphene is used in its metal form in our calculations. The geometrically optimized relaxed structural parameters are (a = b = 3.308 Å, c = 11.961 Å), (a = b = 3.156 Å, c = 12.440 Å), and (a = b = 2.450 Å, c = 7.286 Å) for Gr‐MoS2, MoS2, and Gr, respectively. The energy bandgap of the three hexagonal crystals is induced in the pressure limit (0–40) GPa. Pressure give rise to bandgap in Gr, and does not affect the MoS2 bandgap significantly, whereas the minimal bandgap for Gr‐MoS2 changes drastically under pressure. The pressure‐dependent elastic moduli and related mechanical characteristics are reported. The optical properties are calculated throughout incident photon energies (0–40 eV). Most of the reported properties show a relative agreement with the cited literature. However, there are no experimental nor reliable theoretical results for the Gr‐MoS2 heterostructure, and to the best of the author's knowledge many of the results of Gr‐MoS2 are reported first in this work.

High-Temperature Quantum Hall Effect in Graphite-Gated Graphene Heterostructure Devices with High Carrier Mobility.

Since the discovery of the quantum Hall effect in 1980, it has attracted intense interest in condensed matter physics and has led to a new type of metrological standard by utilizing the resistance quantum. Graphene, a true two-dimensional electron gas material, has demonstrated the half-integer quantum Hall effect and composite-fermion fractional quantum Hall effect due to its unique massless Dirac fermions and ultra-high carrier mobility. Here, we use a monolayer graphene encapsulated with hexagonal boron nitride and few-layer graphite to fabricate micrometer-scale graphene Hall devices. The application of a graphite gate electrode significantly screens the phonon scattering from a conventional SiO2/Si substrate, and thus enhances the carrier mobility of graphene. At a low temperature, the carrier mobility of graphene devices can reach 3 × 105 cm2/V·s, and at room temperature, the carrier mobility can still exceed 1 × 105 cm2/V·s, which is very helpful for the development of high-temperature quantum Hall effects under moderate magnetic fields. At a low temperature of 1.6 K, a series of half-integer quantum Hall plateaus are well-observed in graphene with a magnetic field of 1 T. More importantly, the ν = ±2 quantum Hall plateau clearly persists up to 150 K with only a few-tesla magnetic field. These findings show that graphite-gated high-mobility graphene devices hold great potential for high-sensitivity Hall sensors and resistance metrology standards for the new Système International d'unités.

Gate-Tunable Proximity Effects in Graphene on Layered Magnetic Insulators.

The extreme versatility of van der Waals materials originates from their ability to exhibit new electronic properties when assembled in close proximity to dissimilar crystals. For example, although graphene is inherently nonmagnetic, recent work has reported a magnetic proximity effect in graphene interfaced with magnetic substrates, potentially enabling a pathway toward achieving a high-temperature quantum anomalous Hall effect. Here, we investigate heterostructures of graphene and chromium trihalide magnetic insulators (CrI3, CrBr3, and CrCl3). Surprisingly, we are unable to detect a magnetic exchange field in the graphene but instead discover proximity effects featuring unprecedented gate tunability. The graphene becomes highly hole-doped due to charge transfer from the neighboring magnetic insulator and further exhibits a variety of atypical gate-dependent transport features. The charge transfer can additionally be altered upon switching the magnetic states of the nearest CrI3 layers. Our results provide a roadmap for exploiting proximity effects arising in graphene coupled to magnetic insulators.

Bilayer Heterostructure of Boron Nitride and Graphene for Hydrogen Storage: A First-Principles Study

Bilayer Heterostructure of Boron Nitride and Graphene for Hydrogen Storage: A First-Principles Study

Giant ferroelectric polarization in a bilayer graphene heterostructure

At the interface of van der Waals heterostructures, the crystal symmetry and the electronic structure can be reconstructed, giving rise to physical properties superior to or absent in parent materials. Here by studying a Bernal bilayer graphene moiré superlattice encapsulated by 30°-twisted boron nitride flakes, we report an unprecedented ferroelectric polarization with the areal charge density up to 1013 cm−2, which is far beyond the capacity of a moiré band. The translated polarization ~5 pC m−1 is among the highest interfacial ferroelectrics engineered by artificially stacking van der Waals crystals. The gate-specific ferroelectricity and co-occurring anomalous screening are further visualized via Landau levels, and remain robust for Fermi surfaces outside moiré bands, confirming their independence on correlated electrons. We also find that the gate-specific resistance hysteresis loops could be turned off by the other gate, providing an additional control knob. Furthermore, the ferroelectric switching can be applied to intrinsic properties such as topological valley current. Overall, the gate-specific ferroelectricity with strongly enhanced charge polarization may encourage more explorations to optimize and enrich this novel class of ferroelectricity, and promote device applications for ferroelectric switching of various quantum phenomena.

Theoretical prediction on stability, electronic and activity properties of single-atom catalysts anchored graphene and boron phosphide heterostructures

Introduction of graphene-like heterostructures supported single-atom catalysts (SACs) has been considered as a promising strategy for generating sustainable energy sources. By using first-principles calculation, different kinds of single transition metal (TM = Mn, Fe, Co and Ni) atoms anchored boron phosphide (BP-TM) and the effects of added graphene substrate on its electronic property and sensitivity toward the gas reactants are comparably investigated. The graphene support can provide the transferred electron to BP sheet and then regulate the adsorption strength and electronic structure of TM atoms anchored systems (BP-gra-TM). Besides, the adsorption of N2 molecule (parallel and end-on modes) on BP-gra-TM (including pristine and single vacancy (SV) within BP, i.e., BP-gra-SV-TM) are more stable than those on BP-TM and BP-SV-TM sheets, and then induce the change in electronic and magnetic properties of supported systems. Furthermore, the potential catalytic performances of four reactive substrates (BP-TM, BP-gra-TM, BP-SV-TM and BP-gra-SV-TM) are systematically explored for nitrogen reduction reaction (NRR). It is found that the NRR processes on BP-gra-Mn via enzymatic pathways and BP-gra-Fe via distal mechanism have lower limiting potential (0.52 V and 0.63 V) than other ones, indicating that the introduction of graphene can improve the surface activity of BP-TM sheets, which would provide useful guidance to design two-dimensional heterostructure-based SACs with high activity, efficiency and selectivity for the NRR.

Li-decorated borophene–graphene heterostructure under gas adsorption

Two-dimensional materials show great application potentials in gas sensor due to its high surface-to-volume ratio. In this regard, we investigate the adsorption energies of five gases: NH3, NO, NO2, H2S, and SO2, on borophene, with the help of the density functional theory (DFT). The results show the high adsorption capacity of borophene, but gas adsorption can lead to its structural instability with the greatest influence of nitrogen. However, the heterostructure of borophene and graphene (B/G) can inhibit the deformation and optimize the adsorption capacity of borophene. In order to enhance the adsorption of gas molecules on B/G, lithium is doped in the borophene layer, and it shows that the adsorption of sulfur gas can be significantly enhanced. In addition, the transport properties of Li-decorated B/G inhibitors are simulated by non-equilibrium Green’s function method. The graphene composite and Li doping are found to have a great impact on the gas-sensitive properties of borophene, which provides new ideas for the application of borophene in sensors.

Electron Tunneling Enhancement in MoS2/Hexagonal Boron Nitride/Multilayer Graphene Heterostructures by Bubble Formation

Electron Tunneling Enhancement in MoS₂/Hexagonal Boron Nitride/Multilayer Graphene Heterostructures by Bubble Formation

First-principles design of an ultralow frictional interface of a black phosphorus and graphene heterostructure with oxide functionalization and high-pressure conditions

Using first-principles density functional theory (DFT) calculations, we demonstrate that the heterointerface of black phosphorus and graphene (BP/Gr) should be a promising lubricant, especially under high pressure conditions. The ultralow interfacial frictional sliding motion is enabled by graphene oxide (GO) functional groups, such as epoxy and hydroxyl groups. These functional groups significantly modulate the interfacial charge distribution to facilitate interfacial slip. The epoxy functional group enables a reduction in not only the bilayer adhesion but also the shear strength along the armchair direction by approximately 40% compared with that without functionalization. The potential energy surface (PES) calculation shows the possibility of an ultralow friction sliding process in BP/Gr and BP/GO due to the presence of almost frictionless paths. In addition, high-accuracy PES calculations predict an almost frictionless sliding path even under pressure. Interestingly, our calculation shows that the corrugation energy and shear strength of BP/Gr decrease under 10 GPa. We elucidated that the origin of the improved frictional properties under high pressure stems from the unique heterostructure of BP/Gr. These results consistently suggest that BP/Gr is promising lubricants for high-pressure applications, which can be further improved using a design principle of exploring optimal interfaces and functional groups.

Heterointerface effects of lithium intercalation and diffusion in van der Waals heterostructures

The van der Waals gap of two-dimensional (2D) materials provides natural ion diffusion channels for lithium. However, individual 2D materials (such as graphene, ${\mathrm{MoS}}_{2}$, and MXene) cannot offer all the properties needed to maximize the performance for energy storage. Heterostructures, by stacking different 2D materials, are promising for combining the advantages and eliminating the disadvantages of individual 2D materials. ${\mathrm{MoS}}_{2}$-based heterostructures have been prepared for lithium storage, while intercalation and diffusion mechanisms have yet to be elucidated. In the current work, the heterointerface effects on lithium intercalation and diffusion of seven $H$- and distorted $T$ $(\mathit{dT})$-${\mathrm{MoS}}_{2}$-based heterostructures are systematically studied by density functional theory calculations, and highlight the atomistic origin of differences between $H$$(\mathit{dT})$-${\mathrm{MoS}}_{2}$/graphene, $H$$(\mathit{dT})$-${\mathrm{MoS}}_{2}/H$$(\mathit{dT})$-${\mathrm{MoS}}_{2}$, and $H$$(\mathit{dT})$-${\mathrm{MoS}}_{2}$/MXene hetero- and homostructures. From the energetic point of view, the heterointerface makes for an easier phase transition of ${\mathrm{MoS}}_{2}$ to the $\mathit{dT}$ phase in the ${\mathrm{MoS}}_{2}$/graphene heterostructure. Then we discuss the general idea of a rational design of the van der Waals gap for efficient lithium storage and diffusion. Our work opens a new avenue for optimizing the ion diffusion properties of 2D material heterostructures by designing the van der Waals channel for ion diffusion.

Proximity spin-orbit coupling in graphene on alloyed transition metal dichalcogenides

The negligible intrinsic spin-orbit coupling (SOC) in graphene can be enhanced by proximity effects in stacked heterostructures of graphene and transition metal dichalcogenides (TMDCs). The composition of the TMDC layer plays a key role in determining the nature and strength of the resultant SOC induced in the graphene layer. Here, we study the evolution of the proximity--induced SOC as the TMDC layer is deliberately defected. Alloyed $\mathrm{G}/{\mathrm{W}}_{\ensuremath{\chi}}{\mathrm{Mo}}_{1\ensuremath{-}\ensuremath{\chi}}{\mathrm{Se}}_{2}$ heterostructures with diverse compositions ($\ensuremath{\chi}$) and defect distributions are simulated using density functional theory. Comparison with continuum and tight-binding models allows both local and global signatures of the metal-atom alloying to be clarified. Our findings show that, despite some dramatic perturbation of local parameters for individual defects, the low-energy spin and electronic behavior follow a simple effective medium model which depends only on the composition ratio of the metallic species in the TMDC layer. Furthermore, we demonstrate that the topological state of such alloyed systems can be feasibly tuned by controlling this ratio.

Intrinsic dipole-induced self-doping in Janus MXY-based (M = Mo, W; X = S; Y = Se, Te) p–n junctions

Semiconductor p–n junctions are the most rudimentary building block in modern optoelectronic devices. Herein, we report and demonstrate, using first-principle calculations, that an intrinsic dipole induces self-doping in Janus MXY (M = Mo, W; X = S; Y = Se, Te) van der Waals p–n junctions. The stacked dipole moment in Janus MXY homostructures drives the electrons and holes transfer, forming self-doped p-type and n-type regions in the outermost layer, which is in stark distinction from previously applied electric fields. Furthermore, the doping level in G/MXY/G (G = graphene) heterostructures can be modulated by tuning the thickness of the Janus MXY. Taking the G/WSTe/G heterostructure as an example, the charge carrier density could be tunable to when the Janus WSTe thickness is stacked to a double layer. Interestingly, the interface dipole moment caused by the electrons transfer results in an asymmetric doping level in the top graphene and bottom graphene. Our work provides an intrinsic approach for designing self-doped p–n junctions.