Mixed-dimensional Van Der Waals Heterostructures Research Articles

Room-temperature quantum emission from interface excitons in mixed-dimensional heterostructures

The development of van der Waals heterostructures has introduced unconventional phenomena that emerge at atomically precise interfaces. For example, interlayer excitons in two-dimensional transition metal dichalcogenides show intriguing optical properties at low temperatures. Here we report on room-temperature observation of interface excitons in mixed-dimensional heterostructures consisting of two-dimensional tungsten diselenide and one-dimensional carbon nanotubes. Bright emission peaks originating from the interface are identified, spanning a broad energy range within the telecommunication wavelengths. The effect of band alignment is investigated by systematically varying the nanotube bandgap, and we assign the new peaks to interface excitons as they only appear in type-II heterostructures. Room-temperature localization of low-energy interface excitons is indicated by extended lifetimes as well as small excitation saturation powers, and photon correlation measurements confirm antibunching. With mixed-dimensional van der Waals heterostructures where band alignment can be engineered, new opportunities for quantum photonics are envisioned.

Mixed-Dimensional van der Waals Heterostructure (2D ReS2/0D MoS2 Quantum Dots)-Based Broad Spectral Range with Ultrahigh-Responsive Photodetector.

The remarkable properties of two-dimensional (2D) materials have led to significant advancements in photodetection and optoelectronics research. Currently, there are many successful methods that are employed to improve the responsivity of photodetectors, but the limited spectral range of the device remains a limitation. This work demonstrates the development of a mixed-dimensional (2D/0D) hybrid photodetector device fabricated using chemical vapor deposition (CVD)-grown monolayer ReS2 and solution-processed MoS2 quantum dots (QDs). The mixed dimensionality of 2D (ReS2) and zero-dimensional (0D) MoS2 QDs assist in improving the spectral range of the device [ultraviolet (360 nm) to near-infrared (780 nm)]. Further, due to the work function difference between ReS2 and MoS2 QDs, the built-in electric field across the mixed-dimensional interface promotes effective charge separation and migration, resulting in improved responsivities of the device. The calculated responsivities of the fabricated photodetector are 5.4 × 102, 3.3 × 102, and 2.6 × 102 A/W when subjected to visible, UV, and NIR light illumination, which is remarkable when compared to the existing reports on broadband photodetection. The mixed-dimensionality heterostructure coupled with contact engineering paves the way for highly responsive broadband photodetectors for potential applications in security, healthcare, etc.

Electronic structures and quantum capacitance of twisted mixed-dimensional van der Waals heterostructures of graphene/C2H based on tight-binding model

Constructing twisted mixed dimensional graphene-based van der Waals heterostructure (vdWH) is an effective strategy to manipulate the electronic structures and improve the quantum capacitance (C q) of graphene. In this work, mixed dimensional vdWH of graphene/C2H has been proposed owing to similar Dirac semimetal character of one-dimensional C2H with that of graphene. Meanwhile, the influence of twisting angle (θ) and interlayer interaction strength on the electronic structures and the C q of the MD vdWH are systemically explored based on tight binding model. With the fitted hopping integral parameters, it is found that the linear dispersion of the graphene is basically preserved but the bandwidth is decreased with modulating twisting angle and interlayer interaction, and the C q of mixed dimensional vdWH is improved 5–19 times compared with graphene at zero bias. Moreover, the compressed strain could enhance the C q of mixed dimensional vdWH to 74.57 μF cm−2 at zero bias and broaden the low working voltage window of mixed-dimensional vdWH with considerable C q. Our results provide suitable tight-binding model parameters and theoretical guidance for exploring the twisted MD vdWH of graphene/C2H and offer an effective strategy to modulate the electronic structures and the C q of graphene through constructing the MD vdWH.

Ultra-broadband bright light emission from a one-dimensional inorganic van der Waals material

One-dimensional (1D) van der Waals materials have emerged as an intriguing playground to explore novel electronic and optical effects. We report on inorganic one-dimensional SbPS4 nanotube bundles obtained via mechanical exfoliation from bulk crystals. The ability to mechanically exfoliate SbPS4 nanobundles offers the possibility of applying modern 2D material fabrication techniques to create mixed-dimensional van der Waals heterostructures. We find that SbPS4 can readily be exfoliated to yield long (>10 μm) nanobundles with thicknesses that range from 1.3 to 200 nm. We investigated the optical response of semiconducting SbPS4 nanobundles and discovered that upon excitation with blue light, they emit bright and ultra-broadband red light with a quantum yield similar to that of hBN-encapsulated MoSe2. We discovered that the ultra-broadband red light emission is a result of a large ∼1 eV exciton binding energy and a ∼200 meV exciton self-trapping energy, unprecedented in previous material studies. Due to the bright and ultra-broadband light emission, we believe that this class of inorganic 1D van der Waals semiconductors has numerous potential applications, including on-chip tunable nanolasers, and applications that require ultraviolet to visible light conversion, such as lighting and sensing. Overall, our findings open avenues for harnessing the unique characteristics of these nanomaterials, advancing both fundamental research and practical optoelectronic applications.

Mixed-Dimensional van der Waals Heterostructure for High-Performance and Air-Stable Perovskite Nanowire Photodetectors.

An organic-inorganic hybrid perovskite nanowire (NW), CH3NH3PbI3, shows great potential for high-performance photodetectors due to its excellent photoresponse. However, the inefficient carrier collection between the one-dimensional (1D) NWs and metallic electrodes, as well as degradation of the perovskite, limits the viability of the CH3NH3PbI3 NWs for commercial production. Here, we demonstrate a photodetector with a mixed-dimensional van der Waals heterostructure of hexagonal boron nitride (hBN)/graphene (Gr)/1D CH3NH3PbI3, which exhibits excellent responsivity and specific detectivity of up to 558 A/W and 2.3 × 1012 Jones, owing to the improved carrier extraction at the electrical contact between Gr and the NW. As for the atomic encapsulation of hBN, the device is extremely robust and maintains its outstanding performance for more than 2 months when exposed to air. Moreover, benefitting from the 1D geometry of the CH3NH3PbI3 NW, our device is highly sensitive to polarized light. The mixed-dimensional van der Waals heterostructure, hBN/Gr/1D CH3NH3PbI3, would provide a novel idea and protocol for fabricating high-performance and air-stable photoelectronic devices based on organic-inorganic hybrid perovskite NWs.

Direct Epitaxial Growth of InP Nanowires on MoS2 with Strong Nonlinear Optical Response

Mixed-dimensional van der Waals heterostructures are promising for research and technological advances in photonics and optoelectronics. Here we report vapor–liquid–solid (VLS) method-based van der Waals epitaxy of one-dimensional InP nanowires (NWs) directly on two-dimensional MoS2. With optimized growth parameters (V/III ratio, flow rates of precursors, and growth temperature), we successfully grow high-quality InP NWs on MoS2. The density and vertical yield of NWs on MoS2 are significantly high. Due to the unique properties of both materials, we observe strong linear and nonlinear optical responses from the NW/MoS2 heterostructures. Intriguingly, in addition to strong second and third harmonic responses, the mixed-dimensional heterostructures show odd-order high harmonic generation up to seventh order. Our findings can open new possibilities for advancing attosecond physics on a new platform of mixed-dimensional heterostructures.

First-principles studies of the mixed-dimensional van der Waals heterostructures of graphene/MnF4

Constructing a mixed-dimensional (MD) graphene-based van der Waals heterostructure (vdWH) is a viable technique for opening the bandgap and introducing spin polarization in graphene. In this work, we discovered that the adjacent MnF4 can manipulate the carrier doping, bandgap opening, and spin polarization of graphene in the MD vdWH of graphene/MnF4 comprised of two-dimensional (2D) graphene and one-dimensional atomic wire (1D AW) MnF4. By adopting first-principles calculations, we found that graphene can achieve effective p-type doping with the carrier density up to ∼8.89 × 1013–1.03 × 1014 cm−2. With a twisted angle of θ = 10.89° and the compressed distance of dMn-Gra = 2.84 Å, the opened bandgap of graphene (Eg-Gra) achieves 35 and 57 meV for spin-up and spin-down channels due to the sublattice symmetry-breaking in graphene, and the spin splitting energy (ΔES) at the Dirac point reaches 78.7 meV as a result of the graphene–MnF4 interlayer interaction. Remarkably, Eg-Gra is increased to 64 and 79 meV for spin-up and spin-down channels, and ΔES with 202.7 meV is obtained at dMn-Gra = 2.84 Å when the width of 1D MnF4 is doubled. Meanwhile, the n-type Ohmic contact is also realized. Our work underscores the rich interplay in the graphene/MnF4 MD vdWH and provides a significant route with fundamental insights to engineer the spintronic band properties of graphene.

Interface-Driven Assembly of Pentacene/MoS2 Lateral Heterostructures.

Mixed-dimensional van der Waals heterostructures formed by molecular assemblies and 2D materials provide a novel platform for fundamental nanoscience and future nanoelectronics applications. Here we investigate a prototypical hybrid heterostructure between pentacene molecules and 2D MoS2 nanocrystals, deposited on Au(111) by combining pulsed laser deposition and organic molecular beam epitaxy. The obtained structures were investigated in situ by scanning tunneling microscopy and spectroscopy and analyzed theoretically by density functional theory calculations. Our results show the formation of atomically thin pentacene/MoS2 lateral heterostructures on the Au substrate. The most stable pentacene adsorption site corresponds to MoS2 terminations, where the molecules self-assemble parallel to the direction of MoS2 edges. The density of states changes sharply across the pentacene/MoS2 interface, indicating a weak interfacial coupling, which leaves the electronic signature of MoS2 edge states unaltered. This work unveils the self-organization of abrupt mixed-dimensional lateral heterostructures, opening to hybrid devices based on organic/inorganic one-dimensional junctions.

Geometry-asymmetric photodetectors from metal-semiconductor-metal van der Waals heterostructures.

The functional diversities of two-dimensional (2D) material devices with simple architectures are ultimately limited by immature doping techniques. An alternative strategy is to use geometry-asymmetric metal-semiconductor-metal (GA-MSM) structures, which enable the basic functions of semiconductor junctions such as rectification and photovoltaics. Here, the mixed-dimensional van der Waals heterostructures (MDvdWHs) based on the separation and self-assembly of p-type SnS layered nanosheets (NSs) and n-type SnS2 nanoparticles (NPs) are obtained using an aqueous phase exfoliation (APE) method. Due to the surface charge transfer doping, the carrier transport mechanism of devices based on MDvdWHs turns from thermionic field emission (TFE) to thermionic emission (TE), with the rectification factor (Iforward/Ireverse) changing from 0.7 to 3. To further illustrate the experimental results, the generic current transport models of GA-MSM devices have been established based on the TE and TFE mechanisms in which the TE and TFE mechanisms lead to opposite rectification phenomena in good agreement with the experimental results. The GA-MSM devices show a photovoltaic effect with a high responsivity of 35 A W-1 and detectivity of 3.4 × 1011 cm Hz1/2 W-1. This study not only provides a novel strategy to design photovoltaic devices with MDvdWHs, but more importantly, we have established fundamental models for the rectification behavior of GA-MSM devices.

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.

General synthesis of mixed-dimensional van der Waals heterostructures with hexagonal symmetry

The combination of two-dimensional (2D) materials with non-2D materials (quantum dots, nanowires and bulk materials), i.e. mixed-dimensional van der Waals (md-vdW) heterostructures endow 2D materials with remarkable electronics properties. However, it remains a big challenge to synthesize md-vdW heterostructures because of the difference of crystal symmetry between 2D and non-2D materials. Meanwhile, it is difficult to initiate the nucleation due to the lack of chemical active sites on chemical inert surfaces of 2D materials. Herein, we design a general chemical vapor deposition method for synthesizing a broad class of md-vdW heterostructures with well-aligned hexagonal symmetry including MoS2/FeS, MoS2/CoS, MoS2/MnS, MoS2/ZnS, Mo(S x Se1–x )2/ZnS x Se1–x , Mo(S x Se1–x )2/CdS x Se1–x . Combining with DFT calculation, we find that the hexagonal symmetry and the centered clusters of MoS2 and Mo(S x Se1–x )2 nanoflakes are two crucial factors to launch the hexagonally oriented growth and nucleation of non-2D materials on 2D materials. Our discovery opens an opportunity for the versatile hetero-integration of 2D materials and allows systematic investigation of physical properties in a wide variety of md-vdW heterostructures.

Reconfigurable optical memory based on MoS2/QDs mixed-dimensional van der Waals heterostructure

Optical memory unit with the ability to detect and store optical signals is increasingly becoming a crucial part of advanced data communication and image sensing technology. Despite great efforts devoted to develop high-performance optical memory devices based on two-dimensional (2D) material, the photoelectric conversion is still limited to defect-dominant photo-generated carrier trapping/de-trapping process at the interface of 2D materials. Here, a reconfigurable optical memory implanted with photonic programming/electric erasing operation is demonstrated based on MoS2/quantum dots (QDs) mixed-dimensional heterostructure. Unique photoelectric coupling effect between MoS2 and QDs leads to a continuous n-doping on MoS2 channel after light exposure removed, resulting in the generation of persistent photocurrent. Excellent optical memory characteristics such as high programming/erasing ratio, long retention time and stable operation cycles have been achieved and demonstrated to be gate-tunable. Besides, multi-level optical data storage with ten different states is also realized by applying a series of programmable optical signals. Owing to the unique dynamic response of mix-dimensional van der Waals heterostructure to optical and electric signals, our proposed optical memory transistor may pave up a new path to explore photoelectric conversion in low-dimensional system and develop high-performance optoelectronics for a broad range of applications.

Mixed-Dimensional In-Plane Heterostructures from 1D Mo6 Te6 and 2D MoTe2 Synthesized by Te-Flux-Controlled Chemical Vapor Deposition.

Mixed-dimensional van der Waals heterostructures are scientifically important and practically useful because of their interesting exotic properties resulting from their novel hybrid structures. This study reports the composition- and phase-selective fabrication of low-dimensional molybdenum/tellurium (Mo/Te) compounds and the direct synthesis of mixed-dimensional in-plane 1D-2D Mo6 Te6 -MoTe2 heterostructures. The composition and phase of the Mo/Te compounds are controlled by changing the Te atomic flux that is adjusted by the Te temperature. Metallic 1D Mo6 Te6 wires with an intrinsic 1D structure with a diameter of 3-8nm and length of 100-300nm are synthesized to form wire networks under low Te flux conditions, whereas the semiconducting few-layer 2H MoTe2 films preferentially oriented along the <0001>direction are obtained under high Te flux. Under medium Te flux, the mixed-dimensional in-plane 1D-2D Mo6 Te6 -MoTe2 heterostructures are synthesized in which the semiconducting few-layer 2H MoTe2 circular domains are edge-contacted by the metallic 1D Mo6 Te6 wire networks. Furthermore, the present Te-flux-controlled method reveals that the 1D Mo6 Te6 networks change to few-layer MoTe2 films as the Te flux increases. The in-plane 1D-2D Mo6 Te6 -MoTe2 heterostructures synthesized by this method can be considered as advanced edge-contacted 2D semiconductors for high-performance 2D electronics.

Charge Transport: Photomodulation of Charge Transport in All‐Semiconducting 2D–1D van der Waals Heterostructures with Suppressed Persistent Photoconductivity Effect (Adv. Mater. 26/2020)

In article number 2001268, Paolo Samorì and co-workers report the fabrication of all-semiconducting mixed-dimensional van der Waals heterostructures based on monolayer 2D MoS2 and bottom-up synthesized 1D graphene nanoribbons, which efficiently suppress the typical persistent photoconductivity effect of MoS2. Upon interfacing with physisorbed photochromic molecules, the charge transport of MoS2–graphene nanoribbons in field-effect transistors can be significantly photomodulated, which corresponds to multilevel output currents for high-performance optoelectronics.

Molecular-Scale Characterization of Photoinduced Charge Separation in Mixed-Dimensional InSe–Organic van der Waals Heterostructures

Layered indium selenide (InSe) is an emerging two-dimensional semiconductor that has shown significant promise for high-performance transistors and photodetectors. The range of optoelectronic applications for InSe can potentially be broadened by forming mixed-dimensional van der Waals heterostructures with zero-dimensional molecular systems that are widely employed in organic electronics and photovoltaics. Here, we report the spatially resolved investigation of photoinduced charge separation between InSe and two molecules (C70 and C8-BTBT) using scanning tunneling microscopy combined with laser illumination. We experimentally and computationally show that InSe forms type-II and type-I heterojunctions with C70 and C8-BTBT respectively, due to an interplay of charge transfer and dielectric screening at the interface. Laser-excited scanning tunneling spectroscopy reveals a ~0.25 eV decrease in the energy of the lowest unoccupied molecular orbital of C70 with optical illumination. Furthermore, photoluminescence spectroscopy and Kelvin probe force microscopy indicate that electron transfer from InSe to C70 in the type-II heterojunction induces a photovoltage that quantitatively matches the observed downshift in the tunneling spectra. In contrast, no significant changes are observed upon optical illumination in the type-I heterojunction formed between InSe and C8-BTBT. Density functional theory calculations further show that, despite the weak coupling between the molecular species and InSe, the band alignment of these mixed-dimensional heterostructures strongly differs from the one suggested by the ionization potential and electronic affinities of the isolated components. Self-energy corrected density functional theory indicates that these effects are the result of the combination of charge redistribution at the interface and heterogeneous dielectric screening of the electron-electron interactions in the heterostructure. In addition to providing specific insight for mixed-dimensional InSe-organic van der Waals heterostructures, this work establishes a general experimental methodology for studying localized charge transfer at the molecular scale that is applicable to other photoactive nanoscale systems.

Interface-Induced Enhancement of THz Generation and Modulation in Hexagonal Boron Nitride/Si Mixed-Dimensional Van Der Waals Heterostructure

The interfacial effect in mixed-dimensional van der Waals heterostructure can improve the performance of terahertz (THz) components, such as THz emitter and THz modulator, which highlight the development of THz technology. However, this improvement has only been demonstrated in heterostructure based on graphene or small bandgap two-dimensional materials. We present the enhancement for THz generation and modulation from Si by insulating hexagonal boron nitride (h-BN) layer coating. The surface adsorption charges and the interface trapped charges lower the Fermi energy on Si surface, leading to an enlarged built-in electric field. Consequently, THz generation from h-BN/Si demonstrated significant enhancement compared with that by bare Si surface. Furthermore, the enlarged built-in electric field separates the interface photocarriers and reduces the surface recombination rate. The resulting dense charge accumulation raises up the photoconductivity after optical illumination and the enhanced THz modulation at h-BN/Si interface is approximately ten times higher than that by Si surface. These results not only present an efficient THz generation and modulation from the h-BN/Si interface but also pave the way for the deep understanding of interfacial effect in mixed-dimensional van der Waals heterostructures.

Ligand Engineering for Improved All‐Inorganic Perovskite Quantum Dot‐MoS2 Monolayer Mixed Dimensional van der Waals Phototransistor

AbstractCombining intriguing physical properties of 2D crystals and intrinsically remarkable optical properties of halide perovskite quantum dots (QDs), the 0D–2D perovskite QD–based mixed dimensional van der Waals heterostructure (MvdWH) is considered as promising for optoelectronic applications. Even though the interfacial electronic structure of MvdWHs is sufficiently engineered to manipulate the charge carrier behavior, the issue of interfacial charge transfer efficiency originating from the residue ligands that are inevitably introduced by the QDs is still prominently remained. From this perspective, for the first time, a solution‐processed surface ligand density control strategy is demonstrated to balance the QD surface passivation and the interfacial charge carrier extraction and injection efficiency in the 0D–2D MvdWH system. The accurate adjustment of ligand density outside QDs enables the subsequent modulation on interfacial charge carrier transfer efficiency from the aspect of electronic and optoelectronic properties. Furthermore, such kind of ligand engineering toward MvdWH interface is substantially demonstrated in a photogating mechanism–based phototransistor with an improved photoresponsivity as high as 1.13 × 105 A W−1. These results may push forward the evolution of 0D–2D mixed dimensional van der Waals optoelectronics.

Electron transfer and cascade relaxation dynamics of graphene quantum dots/MoS2 monolayer mixed-dimensional van der Waals heterostructures

We realize the efficient modulation of relaxation channels in MoS 2 monolayers and interfacial electron transfer by forming mixed-dimensional VdW heterostructures with graphene quantum dots (GQDs) and find that the cascaded relaxation of hot electrons in GQDs that originate from the quantum confinement effect is the ultimate reason. With femtosecond pump–probe spectroscopy, the biphasic electron injection model and modified rate equations are employed to quantitatively solve the interfacial dynamics. Van der Waals (VdW) heterostructures have emerged as promising materials for atomically thin optoelectronic and photovoltaic applications, where the efficient charge separation after photo-excitation is significant to enhance device performances. However, modulating the interfacial charge transfer is still challenging due to the weak interlayer VdW interaction. Revealing electron transfer and relaxation processes in heterostructures lays foundation to tune the interfacial dynamics and optoelectronic properties. Here, we realize the efficient modulation of relaxation channels in MoS 2 monolayers and interfacial electron transfer by forming mixed-dimensional VdW heterostructures with graphene quantum dots (GQDs). With femtosecond pump–probe spectroscopy, the biphasic electron injection model and modified rate equations are employed to quantitatively solve interfacial transfer rates of mixed-dimensional VdW heterostructures. We find that the cascaded relaxation of hot electrons in GQDs that originates from the quantum confinement effect can intensively affect the interfacial dynamics. Our established model is instructive to optimize performances of future photon-harvesting devices based on mixed low-dimensional heterostructures.

Ultrahigh Vacuum Self-Assembly of Rotationally Commensurate C8-BTBT/MoS2/Graphene Mixed-Dimensional Heterostructures

Mixed-dimensional van der Waals heterostructures combine the advantages of nanomaterials with qualitatively distinct properties such as the extended bandstructures and high charge carrier mobilities of inorganic two-dimensional materials and the discrete orbital energy levels and strong optical absorption of zero-dimensional organic molecules. The synergistic interplay between nanomaterials of distinct dimensionality has enabled a variety of unique applications such as antiambipolar transistors, sensitized photodetectors, and gate-tunable photovoltaics. Because the performance of mixed-dimensional heterostructure devices depends sensitively on the buried interfacial structure, it is of great interest to identify materials and chemistries that naturally form highly ordered heterointerfaces. Toward this end, here we demonstrate ultrahigh vacuum self-assembly of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) monolayers onto epitaxial MoS2/graphene heterostructures. With molecular-resolution scan...

SbSI whisker/PbI2 flake mixed-dimensional van der Waals heterostructure for photodetection

Mixed-dimensional van der Waals heterostructure formed from an individual SbSI whisker and individual PbI2 flake for photodetection.