Visualizing the Anomalous Charge Density Wave States in Graphene/NbSe2 Heterostructures

Tunable Valley Pseudospin and Electron-Phonon Coupling in WSe2/1T-VSe2 Heterostructures

Heterostructure engineering provides versatile platforms for exploring exotic physics and enhancing the device performance through interface coupling. Despite the rich array of physical phenomena presented by heterostructures composed of semiconductor and metal van der Waals materials, significant gaps remain in understanding their optical, thermal, and electronic properties. Here, we demonstrate that the valley pseudospin and electron-phonon coupling in monolayer WSe2 are significantly influenced by interface coupling with 1T-VSe2. The heterointerface alters the relaxation process of valley excitons, leading to a transition in magnetic-field-dependent valley polarization from a linear to a "V" shape. Furthermore, we uncover that enhanced electron-phonon coupling exacerbates variations in exciton and valley exciton behavior with temperature, involving higher phonon energies and a shift from acoustic to optical phonons. These findings highlight a promising pathway to manipulate valley excitons and investigate electron-phonon coupling through van der Waals interface interactions.

Proximity induced charge density wave in a graphene/1T-TaS2 heterostructure

The proximity-effect, whereby materials in contact appropriate each other's electronic-properties, is widely used to induce correlated states, such as superconductivity or magnetism, at heterostructure interfaces. Thus far however, demonstrating the existence of proximity-induced charge-density-waves (PI-CDW) proved challenging. This is due to competing effects, such as screening or co-tunneling into the parent material, that obscured its presence. Here we report the observation of a PI-CDW in a graphene layer contacted by a 1T-TaS2 substrate. Using scanning tunneling microscopy (STM) and spectroscopy (STS) together with theoretical-modeling, we show that the coexistence of a CDW with a Mott-gap in 1T-TaS2 coupled with the Dirac-dispersion of electrons in graphene, makes it possible to unambiguously demonstrate the PI-CDW by ruling out alternative interpretations. Furthermore, we find that the PI-CDW is accompanied by a reduction of the Mott gap in 1T-TaS2 and show that the mechanism underlying the PI-CDW is well-described by short-range exchange-interactions that are distinctly different from previously observed proximity effects.

First-Principles Investigation on the Tunable Electronic Structures and Photocatalytic Properties of AlN/Sc2CF2 and GaN/Sc2CF2 Heterostructures

Heterostructure catalysts are highly anticipated in the field of photocatalytic water splitting. AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are proposed in this work, and the electronic structures were revealed with the first-principles method to explore their photocatalytic properties for water splitting. The results found that the thermodynamically stable AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are indirect semiconductors with reduced band gaps of 1.75 eV and 1.84 eV, respectively. These two heterostructures have been confirmed to have type-Ⅰ band alignments, with both VBM and CBM contributed to by the Sc2CF2 layer. AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures exhibit the potential for photocatalytic water splitting as their VBM and CBM stride over the redox potential of water. Gibbs free energy changes in HER occurring on AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are as low as -0.31 eV and -0.59 eV, respectively. The Gibbs free energy change in HER on the AlN (GaN) layer is much lower than that on the Sc2CF2 surface, owing to the stronger adsorption of H on AlN (GaN). The AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures possess significant improvements in absorption range and intensity compared to monolayered AlN, GaN, and Sc2CF2. In addition, the band gaps, edge positions, and absorption properties of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures can be effectively tuned with strains. All the results indicate that AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are suitable catalysts for photocatalytic water splitting.

Possible charge ordering and anomalous transport in graphene/graphene quantum dot heterostructure

Observations of superconductivity and charge density waves (CDW) in graphene have been elusive thus far due to weak electron-phonon coupling (EPC) interactions. Here, we report a unique observation of anomalous transport and multiple charge ordering phases at high temperatures (T1∼213K,T2∼325K) in a 0D-2D van der Waals (vdW) heterostructure comprising of single layer graphene (SLG) and functionalized (amine) graphene quantum dots (GQD). The presence of functionalized GQD contributed to charge transfer with shifting of the Dirac point ∼ 0.05 eV above the Fermi level (ab initio   simulations) and carrier densityn∼-0.3×1012 cm-2confirming p-doping in SLG and two-fold increase in EPC interaction was achieved. Moreover, we elucidate the interplay between electron-electron and electron-phonon interactions to substantiate high temperature EPC driven charge ordering in the heterostructure through analyses of magnetotransport and weak anti-localization (WAL) framework. Our results provide impetus to investigate strongly correlated phenomena such as CDW and superconducting phase transitions in novel graphene based heterostructures.

Twist-Angle-Dependent Electronic Properties of Exfoliated Single Layer MoS2 on Au(111)

Synthetic materials and heterostructures obtained by the controlled stacking of exfoliated monolayers are emerging as attractive functional materials owing to their highly tunable properties. We present a detailed scanning tunneling microscopy and spectroscopy study of single layer MoS2-on-gold heterostructures as a function of the twist angle. We find that their electronic properties are determined by the hybridization of the constituent layers and are modulated at the moiré period. The hybridization depends on the layer alignment, and the modulation amplitude vanishes with increasing twist angle. We explain our observations in terms of a hybridization between the nearest sulfur and gold atoms, which becomes spatially more homogeneous and weaker as the moiré periodicity decreases with increasing twist angle, unveiling the possibility of tunable hybridization of electronic states via twist angle engineering.

Large-Area Epitaxial Mott Insulating 1T-TaSe2 Monolayer on GaP(111)B

Two-dimensional Mott materials have recently been reported in the dichalcogenide family with high potential for Mottronic applications. Nevertheless, their widespread use as a single or few layers is hampered by their limited device integration resulting from their growth on graphene, a metallic substrate. Here, we report on the fabrication of 1T-TaSe2 monolayers grown by molecular beam epitaxy on semiconducting gallium phosphide substrates. At the nanoscale, the charge density wave reconstruction and a moiré pattern resulting from the monolayer interaction with the substrate are observed by scanning tunneling microscopy. The fully open gap unveiled by tunneling spectroscopy, which can be further manipulated by the proximity of a metal tip, is confirmed by transport measurements from micrometric to millimetric scales, demonstrating a robust Mott insulating phase at up to 400 K.

van der Waals Self-Epitaxial Growth of Inch-Sized Superconducting Niobium Diselenide Films

Ultrathin superconducting films are the basis of superconductor devices. van der Waals (vdW) NbSe2 with noncentrosymmetry exhibits exotic superconductivity and shows promise in superconductor electronic devices. However, the growth of inch-scale NbSe2 films with layer regulation remains a challenge because vdW structural material growth is strongly dependent on the epitaxial guidance of the substrate. Herein, a vdW self-epitaxy strategy is developed to eliminate the substrate driving force in film growth and realize inch-sized NbSe2 film growth with thicknesses from 2.1 to 12.1 nm on arbitrary substrates. The superconducting transition temperature of 5.1 K and superconducting transition width of 0.30 K prove the top homogeneity and quality of superconductivity among all of the synthetic NbSe2 films. Coupled with a large area and substrate compatibility, this work paves the way for developing NbSe2 superconductor electronics.

Modulating Charge-Density Wave Order and Superconductivity from Two Alternative Stacked Monolayers in a Bulk 4Hb-TaSe2 Heterostructure via Pressure

Two-dimensional (2D) van der Waals heterostructures (VDWHs) containing a charge-density wave (CDW) and superconductivity (SC) have revealed rich tunability in their properties, which provide a new route for optimizing their novel exotic states. The interaction between SC and CDW is critical to its properties; however, understanding this interaction within VDWHs is very limited. A comprehensive in situ study and theoretical calculation on bulk 4Hb-TaSe₂ VDWHs consisting of alternately stacking 1T-TaSe₂ and 1H-TaSe₂ monolayers are investigated under high pressure. Surprisingly, the superconductivity competes with the intralayer and adjacent-layer CDW order in 4Hb-TaSe₂, which results in substantially and continually boosted superconductivity under compression. Upon total suppression of the CDW, the superconductivity in the individual layers responds differently to the charge transfer. Our results provide an excellent method to efficiently tune the interplay between SC and CDW in VDWHs and a new avenue for designing materials with tailored properties.

Nuclear Quantum Effects on the Charge-Density Wave Transition in NbX2 (X = S, Se)

Understanding the origin of charge-density wave (CDW) instability is important for manipulating novel collective electronic states. Many layered transition metal dichalcogenides (TMDs) share similarity in the structural and electronic instability, giving rise to diverse CDW phases and superconductivity. It is still puzzling that even isostructural and isoelectronic TMDs show distinct CDW features. For instance, bulk NbSe₂ exhibits CDW order at low temperature, while bulk NbS₂ displays no CDW instability. The CDW transitions in single-layer NbS₂ and NbSe₂ are also different. In the classic limit, we investigate the electron correlation effects on the dimensionality dependence of the CDW ordering. By performing ab initio path integral molecular dynamics simulations and comparative analyses, we further revealed significant nuclear quantum effects in these systems. Specifically, the quantum motion of sulfur anions significantly reduces the CDW transition temperature in both bulk and single-layer NbS₂, resulting in distinct CDW features in the NbS₂ and NbSe₂ systems.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

Temperature-Dependent Phonon Shifts in van der Waals Crystals

In recent years, there has been an explosive increase in the research on van der Waals (vdW) crystals because of their great potential applications in many optoelectronic devices. It is necessary to determine their temperature-dependent lattice vibration characteristics because their thermal and electrical transport are closely related to the anharmonic phonon effect, which will affect the performance of the devices. We review the temperature-dependent Raman spectroscopy of vdW crystals, systematically introduce the thermal behavior of optical phonons, and summarize their shift with temperature. Upon analyzing the theoretical models and summarizing the reported experimental data, it is found that the phonon shifts of vdW crystals have a "quasi-linear" relationship with temperature, which is widely described with first-order temperature (FOT) coefficients obtained through a linear fit. Thus, subsequently, the phonon shifts of monolayer materials, different-thickness crystals, suspended and supported samples, in-plane and out-of-plane modes in the same vdW materials, as well as heterostructures and alloys are discussed through comparative analysis of FOT coefficients.