Vertical Van Der Waals Research Articles

Self-organized twist-heterostructures via aligned van der Waals epitaxy and solid-state transformations

Vertical van der Waals (vdW) heterostructures of 2D crystals with defined interlayer twist are of interest for band-structure engineering via twist moiré superlattice potentials. To date, twist-heterostructures have been realized by micromechanical stacking. Direct synthesis is hindered by the tendency toward equilibrium stacking without interlayer twist. Here, we demonstrate that growing a 2D crystal with fixed azimuthal alignment to the substrate followed by transformation of this intermediate enables a potentially scalable synthesis of twisted heterostructures. Microscopy during growth of ultrathin orthorhombic SnS on trigonal SnS2 shows that vdW epitaxy yields azimuthal order even for non-isotypic 2D crystals. Excess sulfur drives a spontaneous transformation of the few-layer SnS to SnS2, whose orientation – rotated 30° against the underlying SnS2 crystal – is defined by the SnS intermediate rather than the substrate. Preferential nucleation of additional SnS on such twisted domains repeats the process, promising the realization of complex twisted stacks by bottom-up synthesis.

Fabrication of a MoS2/Graphene Nanoribbon Heterojunction Network for Improved Thermoelectric Properties

AbstractA number of 2D materials have been developed that have properties different from bulk materials due to the quantum confinement effect. These 2D materials can also form a vertical van der Waals heterojunction at a large interface with other 2D materials, resulting in unique electronic and thermoelectric properties. However, it is difficult to fabricate a van der Waals heterostructure of 2D materials that can provide a sufficient temperature gradient while also forming carrier paths across the vertical heterojunction. Here, a heterojunction network structure constructed of highly conductive sub‐20‐nm graphene nanoribbons (GNRs) arrays stacked on a semiconducting molybdenum disulfide (MoS2) monolayer is suggested to maximize the heterojunction effect on carrier transport. This heterojunction network allows the carriers inevitably pass back and forth between the GNR and the MoS2 through the vertical heterojunction, effectively utilizing the interfacial properties in thermoelectricity. This structure also can modify the band structure by controlling the linewidth of the nanoribbons, or introduce an interlayer at the heterojunctions to enhance the tunneling effect between the two layers, thus significantly improved thermoelectric properties are achieved such as enhanced electrical conductivity of 700 S m−1 and a high power factor of 222 µW m−1 K−1.

Photocurrent Direction Control and Increased Photovoltaic Effects in All-2D Ultrathin Vertical Heterostructures Using Asymmetric h-BN Tunneling Barriers.

Two-dimensional (2D) materials are atomically thick and without out-of-plane dangling bonds. As a result, they could break the confinement of lattice matching, and thus can be freely mixed and matched together to construct vertical van der Waals heterostructures. Here, we demonstrated an asymmetrical vertical structure of graphene/hexagonal boron nitride (h-BN)/tungsten disulfide (WS2)/graphene using all chemical vapor deposition grown 2D materials. Three building blocks are utilized in this construction: conductive graphene as a good alternative for the metal electrode due to its tunable Fermi level and ultrathin nature, semiconducting transition-metal dichalcogenides (TMDs) as an ultrathin photoactive material, and insulating h-BNas a tunneling barrier. Such an asymmetrical vertical structure exhibits a much stronger photovoltaic effect than the symmetrical vertical one without h-BN. By changing the sequence of h-BN in the vertical stack, we could even control the electron flow direction. Also, improvement has been further made by increasing the thickness of h-BN. The photovoltaic effect is attributed to different possibilities of excited electrons on TMDs to migrate to top and bottom graphene electrodes, which is caused by potential differences introduced by an insulating h-BN layer. This study shows that h-BN could be effectively used as a tunneling barrier in the asymmetrical vertical heterostructure to improve photovoltaic effect and control the electron flow direction, which is crucial for the design of other 2D vertical heterostructures to meet various needs of electronic and optoelectronic devices.

Enhanced Quantum Efficiency in Vertical Mixed-Thickness n-ReS2/p-Si Heterojunction Photodiodes

We fabricated few-layer, multilayer, and mixed-thickness rhenium disulfide (ReS2) based on a vertical van der Waals n–p junction for photosensing applications. ReS2 flake deposition onto a p+2Si substrate led to the formation of a n–p heterojunction with rectifying characteristics and good photosensing ability under reverse bias. A thin ReS2 layer with a Si heterojunction showed weak photosensing performance with a fast response, whereas a thick multilayer ReS2/Si showed an improvement in photocurrent, but an overall degradation of the response time. To overcome the trade-off between responsivity and speed, a mixed-thickness ReS2/Si was fabricated. This heterojunction was found to exhibit the best photoresponse, with a short response time and high quantum efficiency. A high photoresponsivity (at 3 V) of ∼33.47 A/W at a high-speed operation of 80 μs was recorded, making this one of the fastest reported transition metal dichalcogenides with silicon photodiodes with high responsivity. The heterointerface of Si with thickness-independent direct-bandgap ReS2 of mixed thicknesses enabled more gain related to photogenerated carrier trapping, resulting in the observed high photoresponsivity and fast (μs) response. This work demonstrates that a mixture of different thicknesses of ReS2-based n–p junctions results in improved photoresponsivity and speed in optoelectronics and sensor applications.

Direction and strain controlled anisotropic transport behaviors of 2D GeSe-phosphorene vdW heterojunctions

Vertical van der Waals (vdW) heterostructures made up of two or more 2D monolayer materials provide new opportunities for 2D devices. Herein, we study the electronic transport properties of vertical integration of 2D GeSe-phosphorene(GeSe–BP) heterostructure, using the nonequilibrium Green’s function formalism combined with the density-functional theory. The results reveal that the directional dependency and strain tunable transport anisotropic behavior appears in GeSe/BP-stacking vdW heterostructures. The current–voltage (I–V) characteristics indicate that the electric current propagates more easily through the perpendicular buckled direction (Y) than the linear atomic chain direction (X) in the low bias regime regardless of the GeSe–BP stacking, which is supported by the underlying electronic structures along Γ–Y and Γ–X lines. The anisotropic transmission spectra indicate an over 105 on/off ratio between the IY and IX in GeSe–BP systems. This anisotropic transmission behavior of 2D GeSe–BP heterojunction is regardless of the considered layer distances. The similar situation can also be found in the I–V characteristics of GeSe–BP nano-device after applying a strain, and a charming behavior that the transport gap can be minished obviously when applied a compressed strain on the perpendicular y-direction or the stretched strain on the x-direction. Moreover, an intriguing semiconductor-metal transition induces by applying the in-plain strain along the y-direction. These results imply that the GeSe–BP nanojunctions may be a promising application in futuristic nano-switching materials.

Anisotropic magnetoconductance and Coulomb blockade in defect engineered Cr2Ge2Te6 van der Waals heterostructures

We demonstrate anisotropic tunnel magnetoconductance by controllably engineering charging islands in the layered semiconducting ferromagnet ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$. This is achieved by assembling vertical van der Waals heterostructures comprised of graphene electrodes separated by crystals of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$. Carefully applying vertical electric fields in the region of $(E\ensuremath{\sim}25$--50 mV/nm) across the ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ causes its dielectric breakdown at cryogenic temperatures. This breakdown process has the effect of introducing subgap defect states within the otherwise semiconducting ferromagnetic material. Low-temperature electron transport through charging islands reveals Coulomb blockade behavior with a strongly gate-tuneable anisotropic magnetoconductance, which persists up to $T\ensuremath{\sim}60$ K. We report average tunnel magnetoresistance values of $100%$. This work opens new avenues and material systems for the development of nanometer-scale electrically controlled spintronic devices.

A two-dimensional vertical van der Waals heterostructure based on g-GaN and Mg(OH)2 used as a promising photocatalyst for water splitting: A first-principles calculation

In this study, based on first-principles calculation, the structural, electronic, interfacial, and optical properties of two-dimensional (2D) semiconductor vertical heterostructure constructed by g-GaN and Mg(OH)2 are addressed. The g-GaN/Mg(OH)2 heterostructure is discovered to be formed by van der Waals (vdW) forces and possesses a type-II band structure which can promote the separation of photogenerated electron–holes constantly. At the same time, the calculated band edge positions of the heterostructure are decent to induce the oxidation and reduction reactions for water splitting at pH 0. Gibb's free energy change in the redox reaction for the g-GaN/Mg(OH)2 vdW heterostructure is further investigated that the heterostructure can act as a suitable catalyst in hydrogen evolution reaction and oxygen evolution reaction for water splitting. The charge-density difference and the potential drop are calculated across the interface of the g-GaN/Mg(OH)2 vdW heterostructure, and the potential drop can induce a large built-in electric field, which is also a boost to prevent the recombination of the photogenerated charges. Finally, the applied external biaxial strain is studied that it can improve the optical absorption performance of the g-GaN/Mg(OH)2 vdW heterostructure. This study provides a possibility of method to design the 2D vdW heterostructure as a photocatalyst to decompose water.

Schottky barrier modulation of a GaTe/graphene heterostructure by interlayer distance and perpendicular electric field

Two-dimensional materials have recently been the focus of extensive research. Graphene-based vertical van der Waals heterostructures are expected to design and fabricate novel electronic and optoelectronic devices. Monolayer gallium telluride is a graphene-like nanosheet synthesized in experiment. Here, the electronic properties of GaTe/graphene heterostructures are investigated under the interlayer coupling and the applied perpendicular electric field. The results show that the electronic properties of GaTe and graphene are preserved, and the energy bandgap of graphene is opened to 13.5 meV in the GaTe/graphene heterostructure. It is found that the n-type Schottky contact is formed in the GaTe/graphene heterostructure, which can be tuned by the interlayer coupling, and the applied electric field. Moreover, a transformation from n-type to p-type Schottky contact is observed when the interlayer distance is smaller than 3.15 Å or the applied electric field is larger than 0.05 V Å−1. These properties are fundamental to the design of new Schottky nanodevices based on the GaTe/graphene heterostructure.

Electronic Transport and Thermopower in 2D and 3D Heterostructures—A Theory Perspective

AbstractThe impact of interfaces and heterojuctions on the electronic and thermoelectric transport properties of materials is discussed herein. Recent progress in understanding electronic transport in heterostructures of 2D materials ranging from graphene to transition metal dichalcogenides, their homojunctions (grain boundaries), lateral heterojunctions (such as graphene/MoS2 lateral interfaces), and vertical van der Waals heterostructures is reviewed. Work on thermopower in 2D heterojunctions, as well as their applications in creating devices such as resonant tunneling diodes (RTDs), is also discussed. Last, the focus turns to work in 3D heterostructures. While transport in 3D heterostructures has been researched for several decades, here recent progress in theory and simulation of quantum effects on transport via the Wigner and non‐equilibrium Green's functions approaches is reviewed. These simulation techniques have been successfully applied toward understanding the impact of heterojunctions on transport properties and thermopower, which finds applications in energy harvesting, and electron resonant tunneling, with applications in RTDs. In conclusion, tremendous progress has been made in both simulation and experiments toward the goal of understanding transport in heterostructures and this progress will soon be parlayed into improved energy converters and quantum nanoelectronic devices.

Wrap-Around Core-Shell Heterostructures of Layered Crystals.

Engineered heterostructures create new functionality by integrating dissimilar materials. Combining different 2D crystals naturally produces two distinct classes of heterostructures, vertical van der Waals (vdW) stacks or 2D sheets bonded laterally by covalent line interfaces. When joining thicker layered crystals, the arising structural and topological conflicts can result in more complex geometries. Phase separation during one-pot synthesis of layered tin chalcogenides spontaneously creates core-shell structures in which large orthorhombic SnS crystals are enclosed in a wrap-around shell of trigonal SnS2 , forcing the coexistence of parallel vdW layering along with unconventional, orthogonally layered core-shell interfaces. Measurements of the optoelectronic properties establish anisotropic carrier separation near type II core-shell interfaces and extended long-wavelength light harvesting via spatially indirect interfacial absorption, making multifunctional layered core-shell structures attractive for energy-conversion applications.

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.

Interfacial Interactions and Enhanced Optoelectronic Properties in CsSnI3–Black Phosphorus van der Waals Heterostructures

Vertical Van der Waals (vdW) heterostructures, characterized by two‐dimensional materials and halide perovskites, have drawn extensive attention to differences in novel optoelectronic properties for isolated materials. In this paper, the structural, electronic, and optical properties of vdW heterostructures based on γ‐CsSnI3 and black phosphorus (BP) monolayers are investigated using first‐principle calculations. The calculated results show that the I–Cs are investigated interface heterostructure contacting the BP monolayer has a type‐I band alignment, while the I–Sn interface contacting the BP monolayer heterostructure has a type‐II alignment. In addition, CsSnI3–BP heterostructures show superior optical performance compared to CsSnI3 slabs in visible and ultraviolet spectra. This surprising result is traced to the smaller bandgap of heterostructures compared to that of isolated structures, as well as the inner electric field caused by the potential across the interface, which produces a charge redistribution at the interface and a separation of electron–hole pairs. This work offers a new view to shed light on the interface charge transfer mechanism for hybrid heterostructures with enhanced optical absorption. Therefore, a bandgap‐tunable inorganic perovskite may be attractive for optoelectric applications.

Direct Vapor Growth of 2D Vertical Heterostructures with Tunable Band Alignments and Interfacial Charge Transfer Behaviors.

2D vertical van der Waals (vdW) heterostructures with atomically sharp interfaces have attracted tremendous interest in 2D photonic and optoelectronic applications. Band alignment engineering in 2D heterostructures provides a perfect platform for tailoring interfacial charge transfer behaviors, from which desired optical and optoelectronic features can be realized. Here, by developing a two‐step chemical vapor deposition strategy, direct vapor growth of monolayer PbI2 on monolayer transition metal dichalcogenides (TMDCs) (WS2, WSe2, or alloying WS2(1− x )Se2 x), forming bilayer vertical heterostructures, is demonstrated. Based on the calculated electron band structures, the interfacial band alignments of the obtained heterostructures can be gradually tuned from type‐I (PbI2/WS2) to type‐II (PbI2/WSe2). Steady‐state photoluminescence (PL) and time‐resolved PL measurements reveal that the PL emissions from the bottom TMDC layers can be modulated from apparently enhanced (for WS2) to greatly quenched (for WSe2) compared to their monolayer counterparts, which can be attributed to the band alignment–induced distinct interfacial charge transfer behaviors. The band alignment nature of the heterostructures is further demonstrated by the PL excitation spectroscopy and interlayer exciton investigation. The realization of 2D vertical heterostructures with tunable band alignments will provide a new material platform for designing and constructing multifunctional optoelectronic devices.

Planar and van der Waals heterostructures for vertical tunnelling single electron transistors

Despite a rich choice of two-dimensional materials, which exists these days, heterostructures, both vertical (van der Waals) and in-plane, offer an unprecedented control over the properties and functionalities of the resulted structures. Thus, planar heterostructures allow p-n junctions between different two-dimensional semiconductors and graphene nanoribbons with well-defined edges; and vertical heterostructures resulted in the observation of superconductivity in purely carbon-based systems and realisation of vertical tunnelling transistors. Here we demonstrate simultaneous use of in-plane and van der Waals heterostructures to build vertical single electron tunnelling transistors. We grow graphene quantum dots inside the matrix of hexagonal boron nitride, which allows a dramatic reduction of the number of localised states along the perimeter of the quantum dots. The use of hexagonal boron nitride tunnel barriers as contacts to the graphene quantum dots make our transistors reproducible and not dependent on the localised states, opening even larger flexibility when designing future devices.

Energy Dissipation in Black Phosphorus Heterostructured Devices

AbstractTwo‐dimensional (2D) black phosphorus (BP) has attracted increasing interest for next‐generation solid‐state device applications due to its unique blend of versatile properties. The ultrathin physique and low thermal conductivity (40–20 Wm−1 K−1) of BP make it susceptible to premature Joule breakdown under moderate electric field induced by inefficient and nonhomogeneous energy dissipation. Here, it is reported that the back‐gate BP device suffers Joule breakdown merely under 4 MV m−1 electric field value with the centrally localized fracture. The spatial micro‐Raman spectroscopy confirms uneven thermal spreading in BP channel with the center being 20% hotter than the lateral ends. Furthermore, to mitigate the early breakdown and uneven spreading, vertical van der Waals structure is assembled. The results show that the vertical BP device exhibits 230 times higher field strength and one order enhancement in power sustainability than those of lateral devices due to the integration of thermally favorable constituent materials and formation of the optimal path for self‐heat removal.

Magnetoresistance in Co-hBN-NiFe Tunnel Junctions Enhanced by Resonant Tunneling through Single Defects in Ultrathin hBN Barriers.

Hexagonal boron nitride (hBN) is a prototypical high-quality two-dimensional insulator and an ideal material to study tunneling phenomena, as it can be easily integrated in vertical van der Waals devices. For spintronic devices, its potential has been demonstrated both for efficient spin injection in lateral spin valves and as a barrier in magnetic tunnel junctions (MTJs). Here we reveal the effect of point defects inevitably present in mechanically exfoliated hBN on the tunnel magnetoresistance of Co-hBN-NiFe MTJs. We observe a clear enhancement of both the conductance and magnetoresistance of the junction at well-defined bias voltages, indicating resonant tunneling through magnetic (spin-polarized) defect states. The spin polarization of the defect states is attributed to exchange coupling of a paramagnetic impurity in the few-atomic-layer thick hBN to the ferromagnetic electrodes. This is confirmed by excellent agreement with theoretical modeling. Our findings should be taken into account in analyzing tunneling processes in hBN-based magnetic devices. More generally, our study shows the potential of using atomically thin hBN barriers with defects to engineer the magnetoresistance of MTJs and to achieve spin filtering, opening the door toward exploiting the spin degree of freedom in current studies of point defects as quantum emitters.

In-Plane Homojunctions and Their Dominant Effects on Charge Transport in Vertical van der Waals Heterostructures

We report, for the first time, the direct observation of in-plane (lateral) homojunctions in vertical van der Waals heterostructures and their dominant effect on charge transport properties. Particularly, with an all-van der Waals metal–semiconductor vertical Schottky junction (MoS2/β-In2Se3) as a model system, we use scanning gate microscopy to directly visualize the in-plane homojunction in the MoS2 channel, the formation of which is a result of charge transfer between MoS2 and β-In2Se3, as supported by numerical simulations and electrical transport measurements. Importantly, we find that this gate-tunable in-plane junction controls the vertical Schottky junction characteristics under the forward-bias condition. This is in contrast to the common assumption that the charge transport across the vertical heterojunction interface is the dominant process. Our findings are universally relevant in vertical van der Waals heterostructures where the charge transfer occurs across the interface, providing new insig...

Insight and control of the chemical vapor deposition growth parameters and morphological characteristics of graphene/Mo2C heterostructures over liquid catalyst

Transition metal carbides is a family of materials with many fascinating properties, especially when studied close to the 2D limit. Molybdenum carbide (Mo2C) is a member of this group which has attracted interest especially thanks to its potential application as electrode in energy storage as well as in catalysis. The latest methods development has demonstrated the potential to grow ultrathin Mo2C domains of high quality through chemical vapor deposition (CVD) over melted substrates, together with graphene. This progress provides new routes towards the preparation of vertical Van der Waals (VdW) heterostructures. In the present work we provide a detailed study of the dominant growth mechanisms of pure Mo2C crystals as well as graphene/Mo2C heterostructures. The findings reveal that the methane flow controls the growth of graphene/Mo2C heterostructures or pure Mo2C domains, while the high growth temperature and presence of hydrogen significantly increase the growth rate. Additionally, the presence of graphene serves as a blocking diffusion layer, permitting the growth of ultrathin crystals and increasing their nucleation density. Atomic Force Microscopy (AFM) characterization demonstrates the growth of stacks of crystals which consist of individual crystals with thickness as low as ∼2 nm.

MXene–Silicon Van Der Waals Heterostructures for High‐Speed Self‐Driven Photodetectors

MXenes, or transition metal carbides or nitrides, as an advanced 2D materials have already attracted extensive attention due to their high conductivity and large specific surface area for applications in the field of energy storage. MXenes also have many other advanced properties such as good transmittance and adjustable work function over a large range. However, few works study the properties of MXenes in the field of optoelectronics. Here, the optoelectronic properties of Ti3C2TX (with a work function of 4.37 eV) on n‐type silicon (n‐Si) of vertical van der Waals heterostructures are studied. The Ti3C2TX not only functions as the transparent electrode but also contributes to the separation and transport of photo‐induced carriers. After investigations on the influence of annealing, temperature, illumination, and applied voltage on the performance of Ti3C2TX/n‐Si Schottky junction heterostructures, this study fabricates a self‐driven vertical junction photodetectors with high response and recovery speeds. It is believed that the excellent photoelectric properties of MXenes will attract many researchers' attention to the application of MXenes in the photoelectrical field.

(Invited) Low-Voltage Complementary Electronics from Ion Gel-Gated Vertical Van Der Waals Heterostructures

Isolation of ultrathin layered materials with disparate electronic properties has prompted their integration into van der Waals heterostructures with unique properties for electronics. By employing ion gel dielectrics, we demonstrate here low-voltage complementary circuits comprised of n-type and p-type van der Waals heterojunction vertical field-effect transistors (VFETs). The n-type and p-type VFETs were fabricated from MoS2 and WSe2 layers on graphene, respectively, with gating achieved by a top ion gel dielectric. Due to the high specific capacitance of the ion gel dielectric, the work function of the underlying graphene and consequently the energy barrier for charge injection into the MoS2 and WSe2 layers are widely tuned at low operating voltages. The resulting VFETs possess high on-state current densities (> 3000 A cm-2) and on/off current ratios (> 104) in a narrow voltage window (< 3 V). In addition to enhancing VFET device metrics at reduced operating voltages, the high specific capacitance of the ion gel dielectric allows unprecedented regimes of charge transport to be accessed including p-channel phenomena in graphene-MoS2 vertical heterostructures. Furthermore, since the ion gel-gated VFETs possess low threshold voltages, a low-power complementary logic inverter is demonstrated using n-type and p-type VFETs. Overall, these results demonstrate the feasibility of vertical van der Waals heterostructures for high-performance, low-voltage electronics.