UCLA and the Chinese University of Hong Kong (Shenzhen) recently published a Nature paper: Creating designable quantum solids through layered hybrid superlattices

First author: Wan Zhong (UCLA), Qian Qi (Hong Kong University of Science and Technology) 

Corresponding author: Duan Xiangfeng (UCLA)

Paper DOI: 10.1038/s41586-024-07858-3

Full text at a glance

Although creating new heterogeneous materials is crucial to advancing the development of quantum technology, there are many challenges in terms of material composition differences and interface impurities. Recently, Dr. Wan Zhong, Professor Duan Xiangfeng, and Professor Huang Yu from UCLA collaborated with Professor Qian Qi from the School of Science and Engineering of the Chinese University of Hong Kong (Shenzhen) to publish a prospective article in Nature titled "Layered Hybrid Superlattices as Designable Quantum Solids". This work proposes a method and application prospect for inserting atoms and molecules into the van der Waals gaps of layered two-dimensional atomic crystals to construct layered hybrid superlattices (LHSLs) .

Background

The modern technological revolution relies mainly on two very different material systems: solid-state materials that form the basis of information technology and synthetic molecular systems used in chemical technology and medicine. Solid-state materials are usually prepared at high temperatures (e.g., > 1000 °C) and have thermodynamically determined crystalline order, which is essential for their excellent electronic properties, but usually have limited structural freedom. On the other hand, synthetic molecular systems are usually prepared at relatively low temperatures (e.g., ~100 °C) and show rich structures and functions, but are usually too fragile to be stably integrated into solid-state devices. By constructing organic-inorganic composite systems, the size, symmetry and work of molecules can be used to adjust solid-state materials. This will greatly enrich solid-state systems and enable the realization of completely new artificial solids. However, due to inherent structural differences and different processing conditions, these materials are difficult to prepare by traditional solid-state reactions or epitaxial methods, becoming a global challenge that restricts the further development of organic-inorganic composite systems.

Research content

First, the team summarized and reviewed the preparation methods of various superlattice structures in the past. Compared with traditional methods, in the manufacturing process of LHSLs, atoms or molecules are inserted into the non-bonded layers of two-dimensional atomic crystals to generate alternating crystal atomic layers and molecular layers. This modular assembly is not only flexible, but also allows it to be carried out at room temperature or near room temperature, avoiding the limitations of high-temperature epitaxial growth. In Figure 1, the researchers demonstrated the high-order superlattice manufacturing process from lattice-matched epitaxial growth, stacking assembly to rolling method. The researchers also compared the cross-sectional images of superlattices obtained using traditional methods (Figure 1b-f) and the high-resolution transmission electron microscope images of LHSLs obtained by intercalation method (Figure 1h). These images reveal that molecular intercalation can form highly ordered molecular layers between crystal atomic layers, generating unique structures and long-range order.

Figure 1. High-order superlattice fabrication processes using lattice-matched epitaxial growth, stacking assembly, curling, and intercalation.

The research team proposed that two-dimensional atomic crystals can be used to combine these two very different material systems to create a new type of synthetic solid that is kinetically stable, controllable in properties, and modular. The two-dimensional atomic crystal layers are van der Waals gaps, which allow the insertion of various molecules without destroying the existing covalent bonds, thereby producing rich layered hybrid superlattices (LHSLs). Layered hybrid superlattices have the following significant advantages: (i) The highly ordered two-dimensional crystal templates on both sides of the molecular layer provide a strong driving force to guide the orderly arrangement of molecules; (ii) The van der Waals interaction between the intercalated molecules and the two-dimensional crystal template allows the molecules to have greater mobility, which can search for the lowest energy state and carry out orderly self-assembly; (iii) The two-dimensional atomic crystal provides strong physical protection and electronic interfaces for the molecules, allowing the molecules to be more firmly integrated into solid-state devices. Ultimately, systems with different chemical compositions and quantum properties are woven into an integrated artificial solid material (Figure 2).

Figure 2 Various layered hybrid superlattices

Although organic molecules are considered incompatible with classical quantum physics that requires an ultra-clean environment, the robustness of different topological systems has demonstrated their highly defect-tolerant characteristics. Through rational molecular design and multifunctional integration strategies, organic-inorganic hybrid systems can introduce molecular complexity into solid-state systems while having rich quantum properties and a stable solid-state environment to realize and utilize unconventional electronic states. Experimentally, unique quantum phenomena including controllable energy gaps, strong second harmonic generation, chiral spin selection effects, and unconventional superconductivity have been observed in different layered hybrid superlattice systems (Figure 3). By selecting specific molecular layers, researchers can achieve emerging quantum effects such as low-temperature superconductivity and chiral regulation, providing a potential material platform for quantum computing and information technology.

Figure 3 Study on the quantum properties of layered hybrid superlattices

Furthermore, by using different materials and intercalation molecules to construct LHSLs , quantum properties such as spin, superconductivity and chirality can be introduced to regulate the band structure of two-dimensional materials, which are difficult to achieve in conventional materials. For example, inserting a molecular layer with magnetism or chirality can produce controllable magnetic and electronic properties, or inserting different two-dimensional materials to form a three-dimensional heterojunction to achieve room temperature Bose-Einstein condensation. This has broad application potential in the fields of quantum information and quantum computing. It is also expected to make breakthroughs in the development and physical property research of high-temperature magnetic semiconductor materials, ferroelectric Rashba semiconductor materials, and room temperature exciton systems (Figure 4).

Fig. 4 Emerging physical properties and band-modulation properties in LHSLs

On the other hand, the self-assembled molecular layer provides a two-dimensional artificial barrier in the xy dimension, and the alternating two-dimensional atomic crystal and molecular layer provide a barrier change in the z-axis direction. Therefore, the composite superlattice is a platform that can realize the regulation of three-dimensional artificial barriers (Figure 5). In view of the many exotic phenomena observed in materials with one-dimensional artificial barrier regulation (two-dimensional electron gas in semiconductor superlattices, quantum cascade lasers, and high electron mobility transistors) or two-dimensional artificial barrier regulation (adjustable electron-electron correlation, flat band structure, unconventional superconductivity, and Hofstadter butterfly in two-dimensional moiré superlattices) and the unique devices realized, we are full of expectations for layered hybrid superlattices with the potential for three-dimensional artificial barrier regulation. In addition, through innovative intercalation methods, layered hybrid superlattice systems are expected to construct three-dimensional moiré superlattice systems and create richer structures such as Kitaev honeycomb lattices or Kagome lattices by using metal organic framework (MOF) and covalent organic framework (COF) materials. Layered hybrid superlattices utilize diverse molecular structures to periodically regulate the dielectric environment or electrostatic potential barriers of adjacent two-dimensional systems, thereby achieving a control range that cannot be achieved by natural atomic lattices or moiré superlattices, and obtaining exotic quantum properties that are difficult to achieve by other methods.

Systematic study of composite superlattices could lead to transformative technologies, including for more energy-efficient electronics, spin field-effect transistors, spin light-emitting diodes, and a variety of other quantum devices. Given that today's semiconductor technology can generate a wealth of electronic functionality with only a limited number of heterostructures and superlattices, composite superlattices with nearly unlimited combinatorial possibilities will bring endless opportunities.