Twisted van der Waals Quantum Materials: Fundamentals, Tunability, and Applications.

Twisted van der Waals (vdW) quantum materials have emerged as a rapidly developing field of two-dimensional (2D) semiconductors. These materials establish a new central research area and provide a promising platform for studying quantum phenomena and investigating the engineering of novel optoelectronic properties such as single photon emission, nonlinear optical response, magnon physics, and topological superconductivity. These captivating electronic and optical properties result from, and can be tailored by, the interlayer coupling using moiré patterns formed by vertically stacking atomic layers with controlled angle misorientation or lattice mismatch. Their outstanding properties and the high degree of tunability position them as compelling building blocks for both compact quantum-enabled devices and classical optoelectronics. This paper offers a comprehensive review of recent advancements in the understanding and manipulation of twisted van der Waals structures and presents a survey of the state-of-the-art research on moiré superlattices, encompassing interdisciplinary interests. It delves into fundamental theories, synthesis and fabrication, and visualization techniques, and the wide range of novel physical phenomena exhibited by these structures, with a focus on their potential for practical device integration in applications ranging from quantum information to biosensors, and including classical optoelectronics such as modulators, light emitting diodes, lasers, and photodetectors. It highlights the unique ability of moiré superlattices to connect multiple disciplines, covering chemistry, electronics, optics, photonics, magnetism, topological and quantum physics. This comprehensive review provides a valuable resource for researchers interested in moiré superlattices, shedding light on their fundamental characteristics and their potential for transformative applications in various fields.

Extraordinary Nonlinear Optical Interaction from Strained Nanostructures in van der Waals CuInP2S6.

Local strain engineering and structural modification of 2D materials furnish benevolent control over their optoelectronic properties and provide an exciting approach to tune light-matter interaction in layered materials. Application of strain at the nanoscale is typically obtained through permanently deformed nanostructures such as nanowrinkles, which yield large band gap modulation, photoluminescence enhancement, and surface potential. Ultrathin transition metal dichalcogenides (TMDs) have been greatly analyzed for such purposes. Herein, we extend strain-induced nanoengineering to an emerging 2D material, CuInP2S6 (CIPS), and visualize extraordinary control over nonlinear light-matter interaction. Wrinkle nanostructures exhibit ∼160-fold enhancement in second harmonic generation (SHG) compared to unstrained regions, which is additionally influenced by a change in the dielectric environment. The SHG enhancement was significantly modulated by the percentage of applied strain which was numerically estimated. Furthermore, polarization-dependent SHG revealed quenching and enhancement in the parallel and perpendicular directions, respectively, due to the direction of the compressive vector. Our work provides an important advancement in controlling optoelectronic properties beyond TMDs for imminent applications in flexible electronics.

Direct observation of contact resistivity for monolayer TMD based junctions via PL spectroscopy.

Monolayer transition metal dichalcogenides (mTMDs) possess a direct band gap and strong PL emission that is highly sensitive to doping level and interfaces, laying the foundation for investigating the contact between mTMD and metal via PL spectroscopy. Currently, electrical methods have been utilized to measure the contact resistance (RC), but they are complicated, time-consuming, high-cost and suffer from inevitable chemical disorders and Fermi level pinning. In addition, previously reported contact resistances comprise both Schottky barrier and tunnel barrier components. Here, we report a simple, rapid and low-cost method to study the tunnel barrier dominated contact resistance of mTMD based junctions through PL spectroscopy. These junctions are free from chemical disorders and Fermi level pinning. Excluding the Schottky barrier component, solely tunnel barrier dominated contact resistances of 1 L MoSe2/Au and 1 L MoSe2/graphene junctions were estimated to be 147.8 Ω µm and 54.9 Ω µm, respectively. Density functional theory (DFT) simulations revealed that the larger RC of the former was possibly due to the existence of intrinsic effective potential difference (Φbarrier) between mTMD and metal. Both junctions exhibit an increasing tendency of RC as temperature decreases, which is probably attributed to the thermal expansion coefficient (TEC) mismatch-triggered interlayer spacing (d) increase and temperature-induced doping. Remarkably, a significant change of RC was observed in 1 L MoSe2/Au junctions, which is possibly ascribed to the changes of their orbital overlaps. Our results open new avenues for exploring fundamental metal-semiconductor contact principles and constructing high-performance devices.

Recent Developments in van der Waals Antiferromagnetic 2D Materials: Synthesis, Characterization, and Device Implementation.

Magnetism in two dimensions is one of the most intriguing and alluring phenomena in condensed matter physics. Atomically thin 2D materials have emerged as a promising platform for exploring magnetic properties, leading to the development of essential technologies such as supercomputing and data storage. Arising from spin and charge dynamics in elementary particles, magnetism has also unraveled promising advances in spintronic devices and spin-dependent optoelectronics and photonics. Recently, antiferromagnetism in 2D materials has received extensive attention, leading to significant advances in their understanding and emerging applications; such materials have zero net magnetic moment yet are internally magnetic. Several theoretical and experimental approaches have been proposed to probe, characterize, and modulate the magnetic states efficiently in such systems. This Review presents the latest developments and current status for tuning the magnetic properties in distinct 2D van der Waals antiferromagnets. Various state-of-the-art optical techniques deployed to investigate magnetic textures and dynamics are discussed. Furthermore, device concepts based on antiferromagnetic spintronics are scrutinized. We conclude with remarks on related challenges and technological outlook in this rapidly expanding field.

2D Materials and Heterostructures at Extreme Pressure.

2D materials possess wide-tuning properties ranging from semiconducting and metallization to superconducting, etc., which are determined by their structure, empowering them to be appealing in optoelectronic and photovoltaic applications. Pressure is an effective and clean tool that allows modifications of the electronic structure, crystal structure, morphologies, and compositions of 2D materials through van der Waals (vdW) interaction engineering. This enables an insightful understanding of the variable vdW interaction induced structural changes, structure-property relations as well as contributes to the versatile implications of 2D materials. Here, the recent progress of high-pressure research toward 2D materials and heterostructures, involving graphene, boron nitride, transition metal dichalcogenides, 2D perovskites, black phosphorene, MXene, and covalent-organic frameworks, using diamond anvil cell is summarized. A detailed analysis of pressurized structure, phonon dynamics, superconducting, metallization, doping together with optical property is performed. Further, the pressure-induced optimized properties and potential applications as well as the vision of engineering the vdW interactions in heterostructures are highlighted. Finally, conclusions and outlook are presented on the way forward.

Direct Measurement of Folding Angle and Strain Vector in Atomically Thin WS2 Using Second-Harmonic Generation.

Structural engineering techniques such as local strain engineering and folding provide functional control over critical optoelectronic properties of 2D materials. Local strain engineering at the nanoscale level is practically achieved via permanently deformed wrinkled nanostructures, which are reported to show photoluminescence enhancement, bandgap modulation, and funneling effect. Folding in 2D materials is reported to tune optoelecronic properties via folding angle dependent interlayer coupling and symmetry variation. The accurate and efficient monitoring of local strain vector and folding angle is important to optimize the performance of optoelectronic devices. Conventionally, the accurate measurement of both strain amplitude and strain direction in wrinkled nanostructures requires the combined usage of multiple tools resulting in manufacturing lead time and cost. Here, we demonstrate the usage of a single tool, polarization-dependent second-harmonic generation (SHG), to determine the folding angle and strain vector accurately and efficiently in ultrathin WS2. The folding angle in trilayer WS2 folds exhibiting 1-9 times SHG enhancement is probed through variable approaches such as SHG enhancement factor, maxima and minima SHG phase difference, and linear dichroism. In compressive strain induced wrinkled nanostructures, strain-dependent SHG quenching and enhancement is observed parallel and perpendicular, respectively, to the direction of the compressive strain vector, allowing us to determine the local strain vector accurately using a photoelastic approach. We further demonstrate that SHG is highly sensitive to band-nesting-induced transition (C-peak), which can be significantly modulated by strain. Our results show SHG as a powerful probe to folding angle and strain vector.

µ-Acetato-aqua-µ-(5-bromo-2-{1,3-bis-[2-(5-bromo-2-oxidobenzyl-idene-amino)-eth-yl]imidazolidin-2-yl}phenolato)methano-ldinickel(II) methanol disolvate monohydrate.

The crystal structure of the title compound, [Ni(2)(C(27)H(24)Br(3)N(4)O(3))(CH(3)CO(2))(CH(3)OH)(H(2)O)]·2CH(3)OH·H(2)O contains [L(OAc){(CH(3)OH)Ni}{(H(2)O)Ni}] mol-ecules {H(3)L = 2-(5-bromo-2-hy-droxy-phen-yl)-1,3-bis-[4-(5-bromo-2-hy-droxy-phen-yl)-3-aza-but-3-en-yl]-1,3-imidazolidine} with additional water and two methanol solvent mol-ecules. In this instance, one of the two Ni atoms is coordinated to a water and the other to a methanol mol-ecule. The Ni-O and Ni-N distances, as well as the angles about the metal atoms, show quite regular octa-hedra around the central ions. The Ni-O(phenol)-Ni and Ni-O(acetate)-Ni angles are not similar [95.26 (13) and 97.34 (13)°, respectively], indicating that this subtle solvate exchange induces significant differences in the conformation adopted. The coordinated methanol ligand is involved in an intra-molecular hydrogen bond to the uncoordinated O atom of the bridging acetate ligand, while the coordinated water mol-ecule forms a hydrogen bond with the one of the methanol solvent mol-ecules. The water solvent mol-ecule forms strong hydrogen bonds to both phenolate O atoms. The remaining methanol solvent mol-ecule also forms a hydrogen bond with this solvent water mol-ecule.

µ-Acetato-µ-(5-chloro-2-{1,3-bis[2-(5-chloro-2-oxidobenzylideneamino)ethyl]imidazolidin-2-yl}phenolato)-bis[methanolnickel(II)] methanol monosolvate monohydrate.

The crystal structure shows that the title compound, [Ni(2)(CH(3)CO(2))(C(27)H(24)Cl(3)N(4)O(3))(CH(4)O)(2)]·CH(3)OH·H(2)O, con-tains [Ni(2)L(OAc)(CH(3)OH)(2)] mol-ecules in the unit cell {H(3)L = 5-chloro-2-{1,3-bis[2-(5-chloro-2-oxidobenzylideneimino)-ethyl]imidazolidin-2-yl}phenolate} with water and methanol as solvates. The title compound is a neutral dinuclear compound, in which the L(3-) Schiff base acts as a hepta-dentate ligand, using each one of its N(2)O compartments to coordinate a nickel atom. The acetate anion bridges the two nickel atoms via one O while the distorted octahedral coordination sphere for each nickel atom is completed by a coordinated methanol ligand. One of the coordinated methanol ligands is involved in an intra-molecular hydrogen bond to the uncoordinated O atom of the bridging acetate ligand while the other forms a hydrogen bond with the methanol solvate. The solvate water mol-ecule forms strong hydrogen bonds to both terminal phenolato O atoms. The methanol solvate mol-ecule also forms a hydrogen bond with the water solvate mol-ecule.