糖心视频

New periodic structures known as moir茅 lattices can be observed in two-dimensional (2-D) heterostructures containing layers with slightly different lattice vectors, which can in turn support new topological phenomena. It is therefore important to obtain high-resolution imaging of these moir茅 lattices and superstructures to understand the emerging physics. In a new report now published in Science Advances, Kyunghoon Lee and a team of scientists report the imaging process to view moir茅 lattices and superstructures in graphene-based samples under ambient conditions using with ultrahigh-resolution implementation. While the probe tip of the device maintained a gross radius of 100 nm, the research team achieved a spatial resolution better than 5 nm. This setup allowed direct visualization of moir茅 lattices and the composite super-moir茅. The researchers also showed the artificial synthesis of new superstructures arising from the interplay between diverse layers.

Topological physics and new quantum phenomena with moir茅 lattices

Two-dimensional heterostructures composed of atomically thin layers with slightly different lattice vectors can form moir茅 lattices with a large periodicity due to a large lattice mismatch or a small-angle twist in the structure. Such architectures generate a new length and energy scales in stacked 2-D materials to provide an exciting new platform to engineer in van der Waals heterostructures. Superstructures of moir茅 lattices can be formed when similar lattice structures are stacked together to offer extra flexibility to design new quantum phenomena. It is important to characterize the moir茅 lattice and superstructures in a device configuration to understand and control the rich moir茅 physics in 2-D heterostructures.

Traditionally this can be accomplished with (TEM), (AFM) and (STM) techniques. But most methods require specialized sample preparation protocols that are largely unsuited to observe functional devices. Scanning microwave impedance microscopy (sMIM) is an alternative and attractive moir茅 imaging tool compared to existing methods, which combines the benefit of spatial resolution with high sensitivity of local electrical properties of the device. Lee et al. therefore demonstrated an ultra-high-resolution implementation of sMIM, which they also named uMIM to perform nanoscale imaging of moir茅 lattices and superstructures of various graphene-based devices under ambient conditions.

Ultra-high-resolution scanning microwave impedance microscopy

Using the imaging probe, the team revealed several moir茅 superstructures including a supermodulation of the moir茅 lattice and a new Kagome-like moir茅 structure arising from the interplay between closely aligned twisted graphene and . Such moir茅 superstructures can offer new avenues to engineer quantum phenomena in van der Waals heterostructures. During the experiments, the team used the microscope to probe the local complex . The observed tip-sample admittance depended on the local sample conductivity and the team calculated the real and imaginary uMIM signals (as uMIM-Re and uMIM-Im respectively). The imaginary signal was informative to rapidly assess the local conductivity since it increased monotonically with the sheet conductance of the sample. The new analytical imaging method provided a microwave version of the method. Although unlike the near-field microscope, the researchers performed the experiments at contact mode where the electromagnetic coupling between the tip and sample was highly localized at the apex of the tip.

Proof-of-concept with graphene-based systems

The team showed the capability of the imaging technique by viewing the moir茅 superlattice in (tDBG). They resolved three different domains in the tDBG moir茅 lattice using distinct signals to show the usefulness of the technique to identify fine structures of moir茅 lattices in 2-D heterostructures based on local conductivity. To demonstrate the spatial resolution capability of the method, Lee et al. imaged moir茅 defects along the moir茅 lattice, and resolved the defects with sub-5-nm resolution. This method outperformed other optical near-field microscopes.

The scientists then showed the universal applicability of the method to resolve moir茅 structures in a variety of graphene-based systems. For example, the technique facilitated moir茅 observations in epitaxially grown monolayer graphene/hBN (hexagonal boron nitride) samples, synthesized using . The method also resolved the triangular domains in twisted trilayer graphene (tTG) and twisted double bilayer graphene (tDBG). Apart from conventional moir茅 lattices, the ultra-high sensitivity microscopic method also allowed imaging of moir茅 superstructures from three underlying lattices with different lattice vectors, such as twisted double bilayer graphene on hexagonal boron nitride (BG/BG/hBN). While such heterostructures have been previously imaged with conventional techniques, they remain to be . The topographic images showed modifications of the moir茅 structure, which may lead to a modified electronic spectrum that eventually may need to be included in theoretical calculations of the material's electronic structure.

Investigating other moir茅 superstructures

Lee et al. then used the method to investigate other moir茅 superstructures with desirable physical properties. For instance, the has attracted due to the presence of flat bands and exotic quantum and magnetic phases. However, Kagome lattice crystals are relatively rare in nature, while they can be simulated via an optical superlattice in . The team therefore developed a solid-state Kagome-like moir茅 superlattice in BG/BG/hBN (twisted double bilayer graphene on hexagonal boron nitride) systems and visualized a special moir茅 composite via the imaging technique. The scientists examined the resulting structure in detail and compared it with the expected structure of an ideal Kagome lattice.

Outlook

In this way, Kyunghoon Lee and colleagues extensively demonstrated the use of an ultrahigh resolution scanning microwave impedance microscope (sMIM) as a simple, high-throughput and noninvasive method to characterize moir茅 superlattices and superstructures including moir茅 defects. The team also tailored Kagome superlattices in multilayer stacks of graphene-based van der Waals heterostructures. The superior imaging technique will provide better understanding of the heterostructure design paths to investigate their correlation with quantum phenomena in advanced moir茅 superstructures.

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