[Paper Review] Ultra-high-resolution imaging of moiré lattices and superstructures using scanning microwave impedance microscopy under ambient conditions

Kyunghoon Lee, M. Iqbal Bakti Utama+14 more|arXiv (Cornell University)|Jun 7, 2020

Electronic and Structural Properties of Oxides69 references18 citations

TL;DR

This study demonstrates ultra-high-resolution nanoscale imaging of moiré lattices and superstructures in graphene-based heterostructures using scanning microwave impedance microscopy (SMPM) under ambient conditions. Despite a probe tip radius of ~100 nm, the technique achieves sub-5 nm spatial resolution in local conductivity profiles, enabling direct visualization of moiré patterns in magic-angle twisted double bilayer graphene and complex super-moiré structures, opening pathways for designing artificial moiré superlattices.

ABSTRACT

Two-dimensional heterostructures with layers of slightly different lattice vectors exhibit a new periodic structure known as moire lattices. Moire lattice formation provides a powerful new way to engineer the electronic structure of two-dimensional materials for realizing novel correlated and topological phenomena. In addition, superstructures of moire lattices can emerge from multiple misaligned lattice vectors or inhomogeneous strain distribution, which offers an extra degree of freedom in the electronic band structure design. High-resolution imaging of the moire lattices and superstructures is critical for quantitative understanding of emerging moire physics. Here we report the nanoscale imaging of moire lattices and superstructures in various graphene-based samples under ambient conditions using an ultra-high-resolution implementation of scanning microwave impedance microscopy. We show that, quite remarkably, although the scanning probe tip has a gross radius of ~100 nm, an ultra-high spatial resolution in local conductivity profiles better than 5 nm can be achieved. This resolution enhancement not only enables to directly visualize the moire lattices in magic-angle twisted double bilayer graphene and composite super-moire lattices, but also allows design path toward artificial synthesis of novel moire superstructures such as the Kagome moire from the interplay and the supermodulation between twisted graphene and hexagonal boron nitride layers.

Motivation & Objective

• To achieve nanoscale imaging of moiré lattices and superstructures in two-dimensional heterostructures with unprecedented spatial resolution.
• To overcome the inherent resolution limitations of conventional scanning probe microscopy by enhancing spatial resolution beyond the probe tip size.
• To enable quantitative analysis of electronic structure modulation in moiré superlattices formed by twisted graphene and hexagonal boron nitride.
• To provide a practical, ambient-condition-compatible method for studying moiré physics in 2D materials without cryogenic or ultra-high vacuum requirements.
• To explore the potential for engineering novel artificial moiré superstructures, such as Kagome lattices, through controlled stacking and strain.

Proposed method

• Utilization of an ultra-high-resolution scanning microwave impedance microscopy (SMPM) setup to map local conductivity with sub-5 nm spatial resolution.
• Implementation of SMPM under ambient conditions to eliminate the need for vacuum or cryogenic environments.
• Employment of a conductive atomic force microscopy (c-AFM) tip with a nominal radius of ~100 nm, yet achieving effective resolution below 5 nm through advanced signal processing and tip-sample interaction modeling.
• Application of the technique to various graphene-based heterostructures, including twisted double bilayer graphene and graphene/hBN heterostructures with multiple misaligned lattice vectors.
• Use of phase and amplitude signals from microwave frequency response to extract local impedance and conductivity variations at the nanoscale.
• Analysis of periodic conductivity modulations to identify moiré superstructures and their periodicity, including composite and supermodulated patterns.

Experimental results

Research questions

• RQ1 Can scanning microwave impedance microscopy achieve sub-5 nm spatial resolution in local conductivity measurements despite a ~100 nm probe tip?
• RQ2 How do moiré lattices and superstructures in twisted graphene and hexagonal boron nitride heterostructures manifest in nanoscale conductivity maps under ambient conditions?
• RQ3 To what extent can the interplay between multiple lattice misalignments and strain fields generate complex super-moiré patterns detectable by SMPM?
• RQ4 Can the observed resolution enhancement be leveraged to design and characterize artificial moiré superstructures, such as Kagome lattices?
• RQ5 What is the role of ambient conditions in preserving the resolution and signal fidelity of SMPM measurements in 2D van der Waals heterostructures?

Key findings

• The SMPM technique achieves a spatial resolution in local conductivity profiles better than 5 nm, significantly exceeding the ~100 nm probe tip radius.
• Direct imaging of moiré lattices in magic-angle twisted double bilayer graphene was successfully achieved under ambient conditions.
• Composite super-moiré lattices formed by multiple misaligned lattice vectors in graphene/hBN heterostructures were visualized with high fidelity.
• The method revealed supermodulated conductivity patterns arising from inhomogeneous strain distribution in the heterostructures.
• The technique enables the potential for artificial design of novel moiré superstructures, such as Kagome lattices, through controlled stacking and strain engineering.
• The results demonstrate that ambient-condition SMPM is a viable and powerful tool for quantitative nanoscale studies of moiré physics in 2D materials.