Picosecond energy transfer in a transition metal dichalcogenide-graphene heterostructure revealed by transient Raman spectroscopy.

Intense light­matter interactions and unique structural and electrical properties make van der Waals heterostructures composed by graphene (Gr) and monolayer transition metal dichalcogenides (TMD) promising building blocks for tunneling transistors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics, and quantum light emitting devices (QLEDs), bright and narrow-line emitters using minimal amounts of active absorber material. The performance of such devices is critically ruled by interlayer interactions which are still poorly understood in many respects. Specifically, two classes of coupling mechanisms have been proposed, charge transfer (CT) and energy transfer (ET), but their relative efficiency and the underlying physics are open questions. Here, building on a time-resolved Raman scattering experiment, we determine the electronic temperature profile of Gr in response to TMD photoexcitation, tracking the picosecond dynamics of the G and 2D Raman bands. Compelling evidence for a dominant role of the ET process accomplished within a characteristic time of ∼4 ps is provided. Our results suggest the existence of an intermediate process between the observed picosecond ET and the generation of a net charge underlying the slower electric signals detected in optoelectronic applications.

Excitonic Thermalization Bottleneck in Twisted TMD Heterostructures.

Twisted van der Waals heterostructures show intriguing interface exciton physics, including hybridization effects and emergence of moiré potentials. Recent experiments have revealed that moiré-trapped excitons exhibit remarkable dynamics, where excited states show lifetimes that are several orders of magnitude longer than in monolayers. The origin of this behavior is still under debate. Based on a microscopic many-particle approach, we investigate the phonon-driven relaxation cascade of nonequilibrium moiré excitons in the exemplary MoSe2-WSe2 heterostructure. We track exciton relaxation pathways across different moiré mini-bands and identify the phonon-scattering channels assisting the spatial redistribution of excitons into low-energy pockets of the moiré potential. We unravel a phonon bottleneck in the flat band structure at low twist angles preventing excitons from fully thermalizing into the lowest state, explaining the measured enhanced emission intensity and lifetime of excited moiré excitons. Overall, our work provides important insights into exciton relaxation dynamics in flat-band exciton materials.

Thermally Activated Photoluminescence Induced by Tunable Interlayer Interactions in Naturally Occurring van der Waals Superlattice SnS/TiS2.

van der Waals (vdW) superlattices, comprising different 2D materials aligned alternately by weak interlayer interactions, offer versatile structures for the fabrication of novel semiconductor devices. Despite their potential, the precise control of optoelectronic properties with interlayer interactions remains challenging. Here, we investigate the discrepancies between the SnS/TiS2 superlattice (SnTiS3) and its subsystems by comprehensive characterization and DFT calculations. The disappearance of certain Raman modes suggests that the interactions alter the SnS subsystem structure. Specifically, such structural changes transform the band structure from indirect to direct band gap, causing a strong PL emission (∼2.18 eV) in SnTiS3. In addition, the modulation of the optoelectronic properties ultimately leads to the unique phenomenon of thermally activated photoluminescence. This phenomenon is attributed to the inhibition of charge transfer induced by tunable intralayer strains. Our findings extend the understanding of the mechanism of interlayer interactions in van der Waals superlattices and provide insights into the design of high-temperature optoelectronic devices.

Interlayer Charge Transfer Induced Electrical Behavior Transition in 1D AgI@sSWCNT van der Waals Heterostructures.

The emergence of one-dimensional van der Waals heterostructures (1D vdWHs) opens up potential fields with unique properties, but precise synthesis remains a challenge. The utilization of mixed conductive types of carbon nanotubes as templates has imposed restrictions on the investigation of the electrical behavior and interlayer interaction of 1D vdWHs. In this study, we efficiently encapsulated silver iodide in high-purity semiconducting single-walled carbon nanotubes (sSWCNTs), forming 1D AgI@sSWCNT vdWHs. We characterized the semiconductor-metal transition and increased the carrier concentration of individual AgI@sSWCNTs via sensitive dielectric force microscopy and confirmed the results through electrical device tests. The electrical behavior transition was attributed to an interlayer charge transfer, as demonstrated by Kelvin probe force microscopy. Furthermore, we showed that this method of synthesizing 1D heterostructures can be extended to other metal halides. This work opens the door for the further exploration of the electrical properties of 1D vdWHs.

Room Temperature, Twist Angle Independent, Momentum Direct Interlayer Excitons in van der Waals Heterostructures with Wide Spectral Tunability.

Interlayer excitons (IXs) in van der Waals heterostructures with static out of plane dipole moment and long lifetime show promise in the development of exciton based optoelectronic devices and the exploration of many body physics. However, these IXs are not always observed, as the emission is very sensitive to lattice mismatch and twist angle between the constituent materials. Moreover, their emission intensity is very weak compared to that of corresponding intralayer excitons at room temperature. Here we report the room-temperature realization of twist angle independent momentum direct IX in the heterostructures of bulk PbI2 and bilayer WS2. Momentum conserving transitions combined with the large band offsets between the constituent materials enable intense IX emission at room temperature. A long lifetime (∼100 ns), noticeable Stark shift, and tunability of IX emission from 1.70 to 1.45 eV by varying the number of WS2 layers make these heterostructures promising to develop room temperature exciton based optoelectronic devices.

Hybrid Moiré Excitons and Trions in Twisted MoTe2-MoSe2 Heterobilayers.

We report experimental and theoretical studies of MoTe2-MoSe2 heterobilayers with rigid moiré superlattices controlled by the twist angle. Using an effective continuum model that combines resonant interlayer electron tunneling with stacking-dependent moiré potentials, we identify the nature of moiré excitons and the dependence of their energies, oscillator strengths, and Landé g-factors on the twist angle. Within the same framework, we interpret distinct signatures of bound complexes among electrons and moiré excitons in nearly collinear heterostacks. Our work provides a fundamental understanding of hybrid moiré excitons and trions in MoTe2-MoSe2 heterobilayers and establishes the material system as a prime candidate for optical studies of correlated phenomena in moiré lattices.

Double Superconducting Dome of Quasi Two-Dimensional TaS2 in Non-Centrosymmetric van der Waals Heterostructure.

Two-dimensional van der Waals heterostructures (2D-vdWHs) based on transition metal dichalcogenides (TMDs) provide unparalleled control over electronic properties. However, the interlayer coupling is challenged by the interfacial misalignment and defects, which hinders a comprehensive understanding of the intertwined electronic orders, especially superconductivity and charge density wave (CDW). Here, by using pressure to regulate the interlayer coupling of non-centrosymmetric 6R-TaS2 vdWHs, we observe an unprecedented phase diagram in TMDs. This phase diagram encompasses successive suppression of the original CDW states from alternating H-layer and T-layer configurations, the emergence and disappearance of a new CDW-like state, and a double superconducting dome induced by different interlayer coupling effects. These results not only illuminate the crucial role of interlayer coupling in shaping the complex phase diagram of TMD systems but also pave a new avenue for the creation of a novel family of bulk heterostructures with customized 2D properties.

First-Principles High-Throughput Inverse Design of Direct Momentum-Matching Band Alignment van der Waals Heterostructures Utilizing Two-Dimensional Indirect Semiconductors.

Two-dimensional (2D) van der Waals (vdW) heterostructures have attracted widespread attention in photocatalysis. Herein, we employ a novel strategy utilizing first-principles high-throughput inverse design of 2D Z-scheme heterojunctions for photocatalysis. This approach is anchored in high-throughput screening conditions, which are fundamentally based on the characteristics of carrier mechanisms influenced significantly by Z-scheme heterojunctions. A pivotal element of our screening process is the integration of the indirect-to-direct bandgap transition with momentum-matching band alignment in k-space, guiding us to combine two 2D indirect bandgap monolayers into direct Z-scheme heterojunctions characterized by pronounced interlayer excitons. Various stacking modes introduce extra and distinct degrees of freedom that can be useful for tuning the properties of heterostructures, encompassing factors such as components, stacking patterns, and sequences. We demonstrate that various stacking modes can facilitate the indirect-to-direct bandgap transition and the emergence of interlayer excitons. These findings provide exciting opportunities for designing Z-scheme heterojunctions in photocatalysis.

Unveiling the Interlayer Interaction in a 1H/1T TaS2 van der Waals Heterostructure.

This study delves into the intriguing properties of the 1H/1T-TaS2 van der Waals heterostructure, focusing on the transparency of the 1H layer to the charge density wave of the underlying 1T layer. Despite the sizable interlayer separation and metallic nature of the 1H layer, positive bias voltages result in a pronounced superposition of the 1T charge density wave structure on the 1H layer. The conventional explanation relying on tunneling effects proves insufficient. Through a comprehensive investigation combining low-temperature scanning tunneling microscopy, scanning tunneling spectroscopy, non-contact atomic force microscopy, and first-principles calculations, we propose an alternative interpretation. The transparency effect arises from a weak yet substantial electronic coupling between the 1H and 1T layers, challenging prior understanding of the system. Our results highlight the critical role played by interlayer electronic interactions in van der Waals heterostructures to determine the final ground states of the systems.

GaN Surface Passivation by MoS2 Coating.

In this study, we investigate the impact of two-dimensional MoS2 coating on the optical properties of surface GaN/AlGaN quantum wells (QWs). A strong enhancement in GaN QW light emission is observed with monolayer-MoS2 coating, yielding luminescence intensity comparable to that from a QW capped by an AlGaN barrier. Our results demonstrate that MoS2, despite its quite different nature from III-nitride semiconductors, acts as an effective barrier for surface GaN QWs and suppresses spatially localized intrinsic surface states. This finding provides novel pathways for efficient III-nitride surface passivation.

Filter-Free UV Photodetectors Based on Unipolar Barrier Van der Waals α-In2Se3/h-BN Heterostructures.

Visible-blind ultraviolet (UV) light detection has a wide application range in scenes like space environment monitoring and medical imaging. To realize miniaturized UV detectors with high performance and high integration ability, new device structures without bulky light filters need to be developed based on advanced mechanisms. Here the unipolar barrier van der Waals heterostructure (UB-vdWH) photodetector is reported that realizes filter-free visible-blind UV detection with good stability, robustness, selectivity, and high detection performance. The UB-vdWH shows a responsivity of 2452 A W-1, a photo on-off ratio of 2.94 × 105 and a detectivity of 1.26 × 1015 Jones as a UV detector, owing to the intentionally designed barrier height that suppresses dark current and photoresponse to visible light during the transport process. The good performance remains intact during 104 test cycles or even under high temperatures, which proves the stability, and robustness of the UB-vdWH, thus shows the huge potential for a wider application range.

Probing the Phase Transition to a Coherent 2D Kondo Lattice.

Kondo lattices are systems with unusual electronic properties that stem from strong electron correlation, typically studied in intermetallic 3D compounds containing lanthanides or actinides. Lowering the dimensionality of the system enhances the role of electron correlations providing a new tuning knob for the search of novel properties in strongly correlated quantum matter. The realization of a 2D Kondo lattice by stacking a single-layer Mott insulator on a metallic surface is reported. The temperature of the system is steadily lowered and by using high-resolution scanning tunneling spectroscopy, the phase transition leading to the Kondo lattice is followed. Above 27 K the interaction between the Mott insulator and the metal is negligible and both keep their original electronic properties intact. Below 27 K the Kondo screening of the localized electrons in the Mott insulator begins and below 11 K the formation of a coherent quantum electronic state extended to the entire sample, i.e., the Kondo lattice, takes place. By means of density functional theory, the electronic properties of the system and its evolution with temperature are explained. The findings contribute to the exploration of unconventional states in 2D correlated materials.

Interfacial Bonding Induced Charge Transfer in Two-Dimensional Amorphous MoO3-x/Graphdiyne Oxide Non-Van der Waals Heterostructures for Dominant SERS Enhancement.

Two-dimensional semiconductor-based nanomaterials have shown to be an effective substrate for Surface-enhanced Raman Scattering (SERS) spectroscopy. However, the enhancement factor (EF) tends to be relatively weak compared to that of noble metals and does not allow for trace detection of molecules. In this work, we report the successful preparation of two-dimensional (2D) amorphous non-van der Waals heterostructures MoO3-x/GDYO nanomaterials using supercritical CO2. Due to the synergistic effect of the localized surface plasmon resonance (LSPR) effect and the charge transfer effect, it exhibits excellent SERS performance in the detection of methylene blue (MB) molecules, with a detection limit as low as 10-14 M while the enhancement factor (EF) can reach an impressive 2.55×1011. More importantly, the chemical bond bridging at the MoO3-x/GDYO heterostructures interface can accelerate the electron transfer between the interfaces, and the large number of defective surface structures on the heterostructures surface facilitates the chemisorption of MB molecules. And the charge recombination lifetime can be proved by a ~1.7-fold increase during their interfacial electron-transfer process for MoO3-x/GDYO@MB mixture, achieving highly sensitive SERS detection.

Cathodoluminescence from interlayer excitons in a 2D semiconductor heterobilayer.

Photoluminescence has widely been used to study excitons in semiconducting transition metal dichalcogenide (MX2) monolayers, demonstrating strong light-matter interactions and locked spin and valley degrees of freedom. In heterobilayers composed of overlapping monolayers of two different MX2, an interlayer exciton can form, with the hole localised in one layer and the electron in the other. These interlayer excitons are long-lived, field-tunable, and can be trapped by moiré patterns formed at small twist angles between the layers. Here we demonstrate that emission from radiative recombination of interlayer excitons can be observed by cathodoluminescence from a WSe2/MoSe2heterobilayer encapsulated in hexagonal boron nitride. The higher spatial resolution of cathodoluminescence, compared to photoluminescence, allows detailed analysis of sample heterogeneity at the 100 s of nm lengthscales over which twist angles tend to vary in dry-transfer fabricated heterostructures.

Boosting the Curie temperature of GaN monolayer through van der Waals heterostructures.

The pursuit of van der Waals (vdW) heterostructures with high Curie temperature and strong perpendicular magnetic anisotropy (PMA) is vital to the advancement of next generation spintronic devices. First-principles calculations are used to study the electronic structures and magnetic characteristics of GaN/VS2vdW heterostructure under biaxial strain and electrostatic doping. Our findings show that a ferromagnetic ground state with a remarkable Curie temperature (477 K), much above room temperature, exists in GaN/VS2vdW heterostructure and 100% spin polarization efficiency. Additionally, GaN/VS2vdW heterostructure still maintains PMA under biaxial strain, which is indispensable for high-density information storage. We further explore the electron, magnetic, and transport properties of VS2/GaN/VS2vdW sandwich heterostructure, where the magnetoresistivity can reach as high as 40%. Our research indicates that the heterostructure constructed by combining the ferromagnet VS2and the non-magnetic semiconductor GaN is a promising material for vdW spin valve devices at room temperature.

Cathodoluminescence emission and electron energy loss absorption from a 2D transition metal dichalcogenide in van der Waals heterostructures.

Nanoscale variations of optical properties in transition metal dichalcogenide (TMD) monolayers can be explored with cathodoluminescence (CL) and electron energy loss spectroscopy (EELS) using electron microscopes. To increase the CL emission intensity from TMD monolayers, the MoSe2flakes are encapsulated in hexagonal boron nitride (hBN), creating van der Waals (VdW) heterostructures. Until now, the studies have been exclusively focused on scanning transmission electron microscopy (STEM-CL) or scanning electron microscopy (SEM-CL), separately. Here, we present results, using both techniques on the same sample, thereby exploring a large acceleration voltage range. We correlate the CL measurements with STEM-EELS measurements acquired with different energy dispersions, to access both the low-loss region at ultra-high spectral resolution, and the core-loss region. This provides information about the weight of the various absorption phenomena including the direct TMD absorption, the hBN interband transitions, the hBN bulk plasmon, and the core losses of the atoms present in the heterostructure. The S(T)EM-CL measurements from the TMD monolayer only show emission from the A exciton. Combining the STEM-EELS and S(T)EM-CL measurements, we can reconstruct different decay pathways leading to the A exciton CL emission. The comparison with SEM-CL shows that this is also a good technique for TMD heterostructure characterization, where the reduced demands on sample preparation are appealing. To demonstrate the capabilities of SEM-CL imaging, we also measured on a SiO2/Si substrate, quintessential in the sample preparation of two-dimensional materials, which is electron-opaque and can only be measured in SEM-CL. The CL-emitting defects of SiO2make this substrate challenging to use, but we demonstrate that this background can be suppressed by using lower electron energy.

Phonon transport in vacancy induced defective stanene/hBN van der Waals heterostructure.

In this study, Non-Equilibrium Molecular Dynamics (NEMD) simulation is employed to investigate the phonon thermal conductivity (PTC) of Sn/hBN van der Waals heterostructures with different vacancy-induced defects. We deliberately introduce three types of vacancies in Sn/hBN bilayer point vacancies, bivacancies, and edge vacancies at various concentrations ranging from 0.25% to 2%, to examine their effects on PTC across temperatures from 100 K to 600 K. The key findings of our work are (i) PTC declines monotonically with increasing vacancy concentration for all types of vacancies, with a maximum reduction of ∼62% observed at room temperature compared to its pristine form. (ii) The position of defects has an impact on PTC, with a larger decrease observed when defects are present in the hBN layer and a smaller decrease when defects are in the Sn layer. (iii) The type of vacancy also influences PTC, with point vacancies causing the most substantial reduction, followed by bivacancies, and edge vacancies having the least effect. A 2% defect concentration results in a ∼62% decrease in PTC for point vacancies, ∼51% for bivacancies, and ∼32% for edge vacancies. (iv) Finally, our results indicate that for a given defect concentration, PTC decreases as temperature increases. The impact of temperature on thermal conductivity is less pronounced compared to the effect of vacancies for the defective Sn/hBN bilayer. The presence of vacancies and elevated temperatures enhance phonon-defect and phonon-phonon scattering, leading to changes in the phonon density of states (PDOS) profile and the distribution of phonons across different frequencies of Sn/hBN bilayer, thus affecting its thermal conductivity. This work offers new insights into the thermal behavior of vacancy-filled Sn/hBN heterostructures, suggesting potential pathways for modulating thermal conductivity in bilayer van der Waals heterostructures for applications in thermoelectric, optoelectronics, and nanoelectronics in future.

Van der Waals interaction affects wrinkle formation in two-dimensional materials.

Nonlinear mechanics of solids is an exciting field that encompasses both beautiful mathematics, such as the emergence of instabilities and the formation of complex patterns, as well as multiple applications. Two-dimensional crystals and van der Waals (vdW) heterostructures allow revisiting this field on the atomic level, allowing much finer control over the parameters and offering atomistic interpretation of experimental observations. In this work, we consider the formation of instabilities consisting of radially oriented wrinkles around mono- and few-layer "bubbles" in two-dimensional vdW heterostructures. Interestingly, the shape and wavelength of the wrinkles depend not only on the thickness of the two-dimensional crystal forming the bubble, but also on the atomistic structure of the interface between the bubble and the substrate, which can be controlled by their relative orientation. We argue that the periodic nature of these patterns emanates from an energetic balance between the resistance of the top membrane to bending, which favors large wavelength of wrinkles, and the membrane-substrate vdW attraction, which favors small wrinkle amplitude. Employing the classical "Winkler foundation" model of elasticity theory, we show that the number of radial wrinkles conveys a valuable relationship between the bending rigidity of the top membrane and the strength of the vdW interaction. Armed with this relationship, we use our data to demonstrate a nontrivial dependence of the bending rigidity on the number of layers in the top membrane, which shows two different regimes driven by slippage between the layers, and a high sensitivity of the vdW force to the alignment between the substrate and the membrane.

Charge Transfer and Asymmetric Coupling of MoSe2 Valleys to the Magnetic Order of CrSBr.

van der Waals heterostructures composed of two-dimensional (2D) transition metal dichalcogenides and vdW magnetic materials offer an intriguing platform to functionalize valley and excitonic properties in nonmagnetic TMDs. Here, we report magneto photoluminescence (PL) investigations of monolayer (ML) MoSe2 on the layered A-type antiferromagnetic (AFM) semiconductor CrSBr under different magnetic field orientations. Our results reveal a clear influence of the CrSBr magnetic order on the optical properties of MoSe2, such as an anomalous linear-polarization dependence, changes of the exciton/trion energies, a magnetic-field dependence of the PL intensities, and a valley g-factor with signatures of an asymmetric magnetic proximity interaction. Furthermore, first-principles calculations suggest that MoSe2/CrSBr forms a broken-gap (type-III) band alignment, facilitating charge transfer processes. The work establishes that antiferromagnetic-nonmagnetic interfaces can be used to control the valley and excitonic properties of TMDs, relevant for the development of opto-spintronics devices.

Reflective Phase-Contrast for High-Contrast Imaging of van der Waals Heterostructure.

Optical microscopy plays a critical role in the fabrication of two-dimensional (2D) van der Waals heterostructures. An outstanding challenge in conventional microscopy is to visualize transparent 2D layers as well as embedded monolayers in a stacked heterostructure with high optical contrast. Phase-contrast microscopy, first developed by Frits Zernike in the 1930s, leverages the interference effect between specimen scattered light and background light to increase the contrast of transparent specimens. Such phase-contrast microscopy, always in a transmission configuration, revolutionized the study of transparent cellular structures in biology. Here, we develop a versatile reflective phase-contrast microscopy for imaging 2D heterostructures. We employ two spatial light modulators to flexibly control the intensity and phase of the illumination and the reflected light. This reflective phase-contrast microscopy achieves unprecedented high contrast for imaging a transparent 2D monolayer. It also enables direct observation of 2D monolayers embedded inside a thick heterostructure that are "invisible" in conventional microscopy.