Graphene-based Van Der Waals Heterostructures Research Articles

Quantized heat flow in graphene quantum Hall phases: Probing the topological order

Topological quantum numbers are often used to characterize the topological order of phase having protected gapless edge modes when the system is kept in a space with the boundary. The famous examples in this category are the quantized electrical Hall conductance and thermal Hall conductance, which encodes the topological order of integer and fractional quantum Hall states. Here, we review the recent thermal transport study of integer and fractional quantum Hall states realized in graphene-based van der Waals heterostructures.

Probing the Inelastic Electron Tunneling via the Photocurrent in a Vertical Graphene van der Waals Heterostructure.

Inelastic electron tunneling (IET), accompanied by energy transfer between the tunneling charge carriers and other elementary excitations, is widely used to investigate the collective modes and quasiparticles in solid-state materials. In general, the inelastic contribution to the tunneling current is small compared to the elastic part and is therefore only prominent in the second derivative of the tunneling current with respect to the bias voltage. Here we demonstrate a direct observation of the IET by measuring the photoresponse in a graphene-based vertical tunnel junction device. Characteristic peaks/valleys are observed in the bias-voltage-dependent tunneling photocurrent at low temperatures, which barely shift with the gate voltage applied to graphene and diminish gradually as the temperature increases. By comparing with the second-order differential conductance spectra, we establish that these features are associated with the phonon-assisted IET. A simple model based on the photoexcited hot-carrier tunneling in graphene qualitatively explains the response. Our study points to a promising means of probing the low-energy elementary excitations utilizing the graphene-based van der Waals (vdW) heterostructures.

Casimir-Polder interactions of S -state Rydberg atoms with graphene

We investigate the thermal Casimir-Polder (CP) potential of $^{87}\mathrm{Rb}$ atoms in Rydberg $nS$-states near single- and double-layer graphene, and briefly look into the lifetimes near graphene-hexagonal boron nitride (hBN) multilayered structures. The dependence of the CP potential on parameters such as atom-surface distance, temperature, principal quantum number $n$, and graphene Fermi energy are explored. Through large-scale numerical simulations, we show that, in the nonretarded regime, the CP potential is dominated by the nonresonant and evanescent-wave terms which are monotonic, and that, in the retarded regime, the CP potential exhibits spatial oscillations. We identify that the most important contributions to the resonant component of the CP potential come from the $nS\text{\ensuremath{-}}nP$ and $nS\text{\ensuremath{-}}(n\ensuremath{-}1)P$ transitions. Scaling of the CP potential as a function of the principal quantum number and temperature is obtained. A heterostructure comprising hexagonal boron nitride layers sandwiched between two graphene layers is also studied. When the boron nitride layer is sufficiently thin, the CP potential can be weakened by changing the Fermi energy of the top graphene layer. Our study provides insights for understanding and controlling CP potentials experienced by Rydberg atoms near single- and multilayer graphene-based van der Waals heterostructures.

Spin–orbit and exchange proximity couplings in graphene/1T-TaS2 heterostructure triggered by a charge density wave

Proximity-induced fine features and spin-textures of the electronic bands in graphene-based van der Waals heterostructures can be explored from the point of tailoring a twist angle. Here we study spin–orbit coupling and exchange coupling engineering of graphene states in the proximity of 1T-TaS2 not triggering the twist, but a charge density wave (CDW) in 1T-TaS2—a realistic low-temperature phase. Using density functional theory and effective model we found that the emergence of the CDW in 1T-TaS2 significantly enhances Rashba spin–orbit splitting in graphene and tilts the spin texture by a significant Rashba angle—in a very similar way as in the conventional twist-angle scenarios. Moreover, the partially filled Ta d-band in the CDW phase leads to the spontaneous emergence of the in-plane magnetic order that transgresses via proximity from 1T-TaS2 to graphene, hence, simultaneously superimposing along the spin–orbit also the exchange coupling proximity effect. To describe this intricate proximity landscape we have developed an effective model Hamiltonian and provided a minimal set of parameters that excellently reproduces all the spectral features predicted by the first-principles calculations. Conceptually, the CDW provides a highly interesting knob to control the fine features of electronic states and to tailor the superimposed proximity effects—a sort of twistronics without twist.

Valley Hall effect in graphene-like SnX ( X=Si , Ge) buckled monolayers with high charge carrier mobility and low lattice thermal conductivity

A systematic analysis of the electronic and thermal properties of two semiconducting monolayers, SnSi and SnGe, is conducted through the lens of density functional theory. Unlike centrosymmetric graphene, these two monolayers are noncentrosymmetric and buckled in the structure; however, their band dispersion resembles that of graphene with a finite small band gap occurring at the $K$ point of the Brillouin zone. Unlike transition metal dichalcogenide (TMDC) monolayers, the band edges in the studied semiconducting monolayers are devoid of $d$ orbitals and contain $s$ and $p$ orbitals, and thus, the valley spin splitting occurring in these semiconductors is a nontrivial topological effect originating from broken inversion symmetry in the crystal structure, as observed earlier in bilayer graphene or graphene-based van der Waals heterostructures. The electronic properties are found to be tunable via external perturbations like an electric field and biaxial strain. These monolayers are highly flexible, as indicated by the small value of Young's modulus, $<50\phantom{\rule{0.28em}{0ex}}\mathrm{N}/\mathrm{m}$. The charge carrier mobility is calculated within the Boltzmann transport equation and found to be $\ensuremath{\sim}1000\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.28em}{0ex}}{\mathrm{V}}^{\ensuremath{-}}1\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. The phonon dynamics study indicates a very low value of lattice thermal conductivity at room temperature (i.e., ${\ensuremath{\kappa}}_{l}<1.5\phantom{\rule{0.28em}{0ex}}\mathrm{W}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$), which arises from the domination of Umklapp scattering, small group velocity, and low Debye temperature. Such a low value of ${\ensuremath{\kappa}}_{l}$ will facilitate better thermal control than TMDCs or graphene-based devices. In this paper, we indicate that the inherent properties of SnSi and SnGe monolayers, such as valley contrasting properties, high charge carrier mobility, low lattice thermal conductivity, and mechanical flexibility, surpass that of well-known TMDCs (${\mathrm{MoS}}_{2}, {\mathrm{WS}}_{2}$) and boron pnictides (BP, BAs, BSb), thereby motivating their synthesis.

Weak localisation driven by pseudospin-spin entanglement

At low temperatures, quantum corrections, originating from the interference of the many paths an electron may take between two points, tend to dominate the transport properties of two-dimensional conductors. These quantum corrections increase the resistivity in systems such as two-dimensional electron gases (2DEGs) without spin–orbit coupling (SOC), a phenomenon called weak localisation. Including symmetry-breaking SOC leads to a change from weak localisation (WL) to weak anti-localisation (WAL) of the electronic states, i.e. a WL-to-WAL transition. Here, we revisit the Cooperon, the propagator encoding quantum corrections, within the context of ultra-clean graphene-based van der Waals heterostructures with strong symmetry-breaking Bychkov-Rashba SOC to yield two completely counter-intuitive results. Firstly, we find that quantum corrections vary non-monotonically with the SOC strength, a clear indication of non-perturbative physics. Secondly, we observe the exact opposite of that seen in 2DEGs with strong SOC: a WAL-to-WL transition. This dramatic reversal is driven by mode entanglement of the pseudospin and spin degrees of freedom describing graphene’s electronic states. We obtain these results by constructing a non-perturbative treatment of the Cooperon, and observe distinct features in the SOC dependence of the quantum corrections to the electrical conductivity that would otherwise be missed by standard perturbative approaches.

Strain engineering on the electronic properties and interface contact of graphene/GeN3 van der Waals heterostructure

Graphene-based van der Waals (vdW) heterostructures have shown great potential in electronic and optoelectronic nanodevices. Herein, we investigate the electronic property and Schottky barrier of graphene/GeN3 vdW heterostructure by first-principles calculations. It is noted that the electronic natures of graphene and GeN3 monolayers are well preserved in the heterostructure lattice due to the weak vdW interaction. Interestingly, the p-type Schottky contact in graphene/GeN3 heterostructure with a barrier height of 0.21 eV can be effectively tuned by both vertical and horizontal strains. Herein, the carrier concentration in the graphene layer reaches ∼1013 cm−2 level by strain engineering. It is noteworthy that the designed optoelectronic field-effect transistor based on graphene/GeN3 heterostructure exhibits distinguished responsivity of 0.297 AW−1 and impressive external quantum efficiency of 54.5% under illumination based on further non-equilibrium Green's function simulations. Our findings are of utmost significance for the metal–semiconductor vdW contact and corresponding applications in high-performance electronic and optoelectronic devices.

Interlayer coupling and external electric field controllable electronic structures and Schottky contact of HfSeX (X = S, Se)/graphene van der Waals heterostructures

Graphene-based van der Waals heterostructures (vdWHs) owing to their remarkable physical properties and extensive application attract much attention. However, the Schottky barrier height (SBH) at the interface limits the charge injection effect. Here, we investigate the geometry, electronic structures, contact behavior and optical properties of the HfSeX/graphene (HfSeX/G, X = S, Se) vdWHs via first-principles calculations. The effect of interlayer coupling and external electric field (Efield) on the HfSeX/G are examined. Our results show that the Dirac cones of graphene are well preserved in all modes due to weak van der Waals interaction and a small bandgap is opened in graphene. The SBH of n-type Schottky contact types (SCT) HfSe2/G, SHfSe/G and SeHfS/G vdWHs are 0.100, 0.085 and 0.114 eV, respectively. Both the interlayer distance and Efield can induce the n-type SCT of the Janus HfSSe/G to transfer to p-type, and large Efield can cause an Ohmic contact. The HfSeX/G heterostructures enhance light absorption intensity in visible-light region compared with isolate graphene and HfSeX monolayers. The work provides a valuable theoretical guidance for designing Schottky-based devices of the HfSeX/G heterostructures, as well as indicates that the heterostructures have a flexible application prospect in further optoelectronic and field effect transistors devices.

Probing the charge transfer and electron–hole asymmetry in graphene–graphene quantum dot heterostructure

In recent years, graphene-based van der Waals (vdW) heterostructures have come into prominence showcasing interesting charge transfer dynamics which is significant for optoelectronic applications. These novel structures are highly tunable depending on several factors such as the combination of the two-dimensional materials, the number of layers and band alignment exhibiting interfacial charge transfer dynamics. Here, we report on a novel graphene based 0D–2D vdW heterostructure between graphene and amine-functionalized graphene quantum dots (GQD) to investigate the interfacial charge transfer and doping possibilities. Using a combination of ab initio simulations and Kelvin probe force microscopy (KPFM) measurements, we confirm that the incorporation of functional GQDs leads to a charge transfer induced p-type doping in graphene. A shift of the Dirac point by 0.05 eV with respect to the Fermi level (E F) in the graphene from the heterostructure was deduced from the calculated density of states. KPFM measurements revealed an increment in the surface potential of the GQD in the 0D–2D heterostructure by 29 mV with respect to graphene. Furthermore, we conducted power dependent Raman spectroscopy for both graphene and the heterostructure samples. An optical doping-induced gating effect resulted in a stiffening of the G band for electrons and holes in both samples (graphene and the heterostructure), suggesting a breakdown of the adiabatic Born–Oppenheimer approximation. Moreover, charge imbalance and renormalization of the electron–hole dispersion under the additional influence of the doped functional GQDs is pointing to an asymmetry in conduction and carrier mobility.

Chemical vapor deposition of two-dimensional molybdenum nitride/graphene van der Waals heterostructure with enhanced electrocatalytic hydrogen evolution performance

Two-dimensional (2D) ultrathin transition metal nitrides (TMNs) have attracted increasing research interests as a family of rising functional materials for various applications especially energy storage and catalysis. In addition, graphene-based van der Waals (vdW) heterostructures could remarkably promote the catalysis performance compared to the intrinsic TMNs due to the interfacial synergy effect and more efficient charge-transfer kinetics. Here we first report a facile two-step method for synthesis of high-quality molybdenum nitride/graphene (MoN/G) vdW vertical heterostructure through liquid copper-assisted chemical vapor deposition followed by a nitridation process. Graphene not only acts as a Mo diffusion barrier for growth of 2D thin MoN crystals, but also facilitates the charge transport within the heterostructure. Therefore, the as-grown MoN/G heterostructure shows a relatively low overpotential of 155 mV and 259 mV in H2SO4 and KOH solutions, respectively, which indeed exhibits outstanding pH-universal (pH = 0–14) hydrogen evolution reaction (HER) catalytic performance. These results provide a general method for the controllable synthesis of MoN/G heterostructure and promote the further research of TMNs-related catalysts in HER catalysis area.

Effects of electric field on Schottky barrier in graphene and hexagonal boron phosphide heterostructures

Two dimensional graphene-based van der Waals heterostructures have attracted the interest of various researchers due to their unique properties. Based on the contact between metal electrodes and two dimensional materials has a substantial impact on the performance of electronic devices. The interfacial performance and Schottky contact performance with graphene and hexagonal boron phosphide (h-BP) vdW heterostructures are investigated using density functional theory calculations. We investigated eight energy-stable stacking configurations of multilayer graphene and h-BP vdW heterostructures. The different configurations of the h-BP/graphene vdW heterostructures all exhibit n-type Schottky contacts. The band structure of the h-BP in the heterostructures can be modulated under the influence of an external electric field (E field ), thus effectively manipulating the Schottky barrier height (SBH). The n-type SBH increases with the increasing positive E field and eventually the contact transforms into an Ohmic contact, while the Schottky contact transition from n-type to p-type occurs under the negative E field . The data demonstrate that modulating the electronic properties of hexagonal boron phosphide/graphere (h-BP/Gr) vdW heterostructures with E field is a promising approach, which can control the transition from Schottky to Ohmic contacts. The results can provide theoretical support for the design of controlled Schottky nanoelectronic devices. • 2D h-BP/Gr vdW heterostructures possesses a low n-type Schottky barrier height (0.15 eV–0.18 eV). • The transformate heterostructures can be transformed from an n-type Schottky contact to a p-type Schottky contact, then to an Ohmic contact. • The SBH of h-BP/Gr vdW heterostructures can be effectively regulated by an external electric field. • As the number of graphene layers increases the external electric field required for Schottky transition decreases.

Tunable Quantum Tunneling through a Graphene/Bi2Se3 Heterointerface for the Hybrid Photodetection Mechanism.

Graphene-based van der Waals heterostructures are promising building blocks for broadband photodetection because of the gapless nature of graphene. However, their performance is mostly limited by the inevitable trade-off between low dark current and photocurrent generation. Here, we demonstrate a hybrid photodetection mode based on the photogating effect coupled with the photovoltaic effect via tunable quantum tunneling through the unique graphene/Bi2Se3 heterointerface. The tunneling junction formed between the semimetallic graphene and the topologically insulating Bi2Se3 exhibits asymmetric rectifying and hysteretic current–voltage characteristics, which significantly suppresses the dark current and enhances the photocurrent. The photocurrent-to-dark current ratio increases by about a factor of 10 with the electrical tuning of tunneling resistance for efficient light detection covering the major photonic spectral band from the visible to the mid-infrared ranges. Our findings provide a novel concept of using tunable quantum tunneling for highly sensitive broadband photodetection in mixed-dimensional van der Waals heterostructures.

Tunable properties of ZnSe/graphene heterostructure as a promising candidate for photo/electro-catalyst applications

The graphene-based Van der Waals (vdW) heterostructures show great potential for the next generation of photoelectronic nanodevices. Here, we constructed the ZnSe/graphene vdW heterojunction and investigated its structural, electronic, optical, transportable, and catalytic properties by using density functional theory (DFT). The results show that ZnSe is a direct semiconductor with a bandgap of 3.24 eV. The electron mobility of ZnSe is seven times higher than that of the hole, which is beneficial for the photogenerated hole-electron separation. Besides, the n-type Schottky to Ohmic contact transformation can be realized by applying an external electric field of 0.2 V/Å, and the underlying mechanism is discussed. Finally, the potential ability of photo/electrocatalysis for graphene/ZnSe heterostructure and electrocatalysis for ZnSe monolayer is also explored. The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) can spontaneously activate in acid (PH = 1,2) and alkaline (PH > 9) media under light irradiation. The overpotential of oxygen reduction reaction reaches 0.42 eV, which indicates graphene/ZnSe heterostructure would be a potential electrocatalyst. For the HER process of ZnSe monolayer, the overpotential is significantly decreased to 0.23 eV by doping 3d transition metal atom. Consequently, we hope our work can provide useful guidance for designing practical ZnSe-based applications.

Tunable contacts and device performances in graphene/group-III monochalcogenides MX (M = In, Ga; X = S, Se) van der Waals heterostructures

Graphene-based van der Waals (vdW) heterostructures have attracted extensive attention for applications like nanoelectronics. The graphene and group-III monochalcogenide (MX, M = In and Ga and X = S and Se) heterostructures were established herein, and the electronic properties were investigated by the first-principles calculation. These heterostructures form an n-type Schottky contact at the interface and the Schottky barrier height can be modulated by the external strain. With graphene as electrodes, the device performances of the 9 nm MX metal-oxide-semiconductor FETs (MOSFETs) are investigated. Based on the computed tunneling probability and transfer characteristics, the GaSe MOSFET with a graphene electrode stands out with the highest tunneling probability and largest on-off ratio. We believe these results can provide physical insights into designing and fabricating devices based on the graphene and group-III monochalcogenide heterostructures.

Turning electronic performance and Schottky barrier of graphene/β-Si3N4 (0001) heterostructure by external strain and electric field

Graphene-based van der Waals (vdW) heterostructures have currently emerged as a promising application in the construction of next-generation electronic and optoelectronic devices. In this present contribution, through the comprehensive first-principle calculations, the electronic characteristics along with the Schottky barrier of graphene/β-silicon nitride(0001) heterostructure are theoretically investigated, considering external strain and electric fields effects. The results concluded that the electronic performance of both the graphene monolayer and β-Si3N4(0001) surface are perfectly maintained in the heterostructure on account of a weak interlayer vdW force between them. Furthermore, the graphene/β-Si3N4 (0001) heterostructure forms a p-type Schottky contact with the SBH of 0.72 eV, which can be modified by using normal strain or perpendicular electric field. It is found that the heterostructure still maintains a p-type Schottky contact when the interlayer spacing from 2.2 Å to 4.4 Å or when the applied positive electric field is smaller than 0.2 V/Å. Most importantly, a transformation from the p-type Schottky contact to Ohmic one is observed in the heterostructure at an electric field of +0.2 V/Å. The above results are expected to provide a practical guidance for designing and fabrication of the novel nanoelectronic devices based on graphene/β-Si3N4 vdW heterostructure.

Electric gating and interlayer coupling controllable electronic structure and Schottky contact of graphene/ BiI3 van der Waals heterostructure

Graphene-based van der Waals heterostructures have received tremendous interest from both fundamental and experimental studies because they can enhance the properties and expand the possibility of applications of both graphene and two-dimensional materials. Motivated by the successful synthesis of the graphene/$\mathrm{Bi}{\mathrm{I}}_{3}$ heterostructure [Chang et al., Adv. Funct. Mater. 28, 1800179 (2018)]., here, we systematically investigate the electronic structure and interfacial characteristics of this material using first-principles simulations. We find that the structure of the graphene/$\mathrm{Bi}{\mathrm{I}}_{3}$ heterostructure is mainly characterized by weak van der Waals interactions, which keeps the heterostructure feasible. In the ground state, the graphene/$\mathrm{Bi}{\mathrm{I}}_{3}$ heterostructure forms the $n$-type Schottky contact with a barrier of 0.53 eV. The barriers of the Schottky contact can be adjusted by various factors, including interlayer coupling and electric gating. Both the interlayer coupling and electric gating lead to the transformation from the $n$-type Schottky contact to the $p$-type one or to the $n$-type Ohmic contact. These findings demonstrate that graphene/$\mathrm{Bi}{\mathrm{I}}_{3}$ can be considered a promising building block for high-performance photoresponsive optoelectronic devices.

Achieving an Ohmic contact in graphene-based van der Waals heterostructures by intrinsic defects and the inner polarized electric field of Janus AlGaSSe

The Schottky barrier height is mainly affected by the synergistic action of the inner polarization electric field of Janus AlGaSSe and the 2D/2D heterostructure.

Two-dimensional van der Waals graphene/transition metal nitride heterostructures as promising high-performance nanodevices

Graphene-based van der Waals (vdW) heterostructures have attracted much attention because they can enhance the properties of separated materials, possess numerous new phenomena and unusual properties and improve the performance of devices.

Interfacial characteristics, Schottky contact, and optical performance of a graphene/Ga2SSe van der Waals heterostructure: Strain engineering and electric field tunability

Two-dimensional graphene-based van der Waals heterostructures have received considerable interest because of their intriguing characteristics compared with the constituent single-layer two-dimensional materials. Here, we investigate the interfacial characteristics, Schottky contact, and optical performance of $\mathrm{graphene}/{\mathrm{Ga}}_{2}\mathrm{SSe}$ van der Waals (vdW) heterostructure using first-principles calculations. The effects of stacking patterns, electric gating, and interlayer coupling on the interfacial properties of $\mathrm{graphene}/{\mathrm{Ga}}_{2}\mathrm{SSe}$ heterostructures are also examined. Our results demonstrate that the Dirac cone of graphene is well preserved at the $\mathrm{\ensuremath{\Gamma}}$ point in all stacking patterns due to the weak vdW interactions, which keep the heterostructures feasible such that they can be obtained in further experiments. Moreover, depending on the stacking patterns, a small band gap of about 13--17 meV opens in graphene and has a high carrier mobility, indicating that the $\mathrm{graphene}/{\mathrm{Ga}}_{2}\mathrm{SSe}$ heterostructures are potential candidates for future high-speed nanoelectronic applications. In the ground state, the $\mathrm{graphene}/{\mathrm{Ga}}_{2}\mathrm{SSe}$ heterostructures form an $n$-type Schottky contact. The transformation from an $n$-type to a $p$-type Schottky contact or to an Ohmic contact can be forced by electric gating or by varying the interlayer coupling. Our findings could provide physical guidance for designing controllable Schottky nanodevices with high electronic and optical performances.

Doping effect in graphene-graphene oxide interlayer

Interlayer coupling in graphene-based van der Waals (vdW) heterostructures plays a key role in determining and modulating their physical properties. Hence, its influence on the optical and electronic properties cannot be overlooked in order to promote various next-generation applications in electronic and opto-electronic devices based on the low-dimensional materials. Herein, the optical and electrical properties of the vertically stacked large area heterostructure of the monolayer graphene transferred onto a monolayer graphene oxide film are investigated. An effective and stable p-doping property of this structure is shown by comparison to that of the graphene device fabricated on a silicon oxide substrate. Through Raman spectroscopy and density functional theory calculations of the charge transport characteristics, it is found that graphene is affected by sustainable p-doping effects induced from underneath graphene oxide even though they have weak interlayer interactions. This finding can facilitate the development of various fascinating graphene-based heterostructures and extend their practical applications in integrated devices with advanced functionalities.