Phase landscapes in low-dimensional structures

By precisely controlling the phases in materials with reduced dimensionality, the material properties can be tailored, leading to enhanced performance and multifunctionality.

Phase engineering allows for material customization with desired properties and functions. As dimensionality is reduced from the bulk to two-dimensional (2D) and 1D structures, the atomic arrangements are increasingly dominated by defects and are sensitive to external conditions, which largely impacts the electronic, optical, catalytic, mechanical and biological properties of the material. Therefore, precise and efficient phase-engineering techniques are crucial. In this issue of Nature Materials we present a Focus on recent developments in phase-engineering strategies for low-dimensional structures.

Credit: Chaoyang Zhao, independent artist, and Junjie Shan, Nanjing University

Phase control of 2D and 1D materials can be achieved by top-down or bottom-up approaches, or a hybrid of the two. For 2D materials, different phases can be integrated to form complex heterostructures or superlattices. Compared with the top-down methods of exfoliation and stacking, bottom-up synthesis enables scalability and more controllable spatial modulation of chemical compositions and phases. However, growing 2D materials and their heterostructures is fundamentally challenging owing to high anisotropy during growth, the lack of intrinsic substrates and the inability to precisely control nucleation and growth kinetics. In a Review Article in this issue, Jia Li and colleagues discuss these challenges and strategies to synthesize 2D monolayers, various heterostructures as well as moiré superlattices. On this basis, future devices and applications will benefit from innovative combinations of various phases for advanced functionalities and from the development of techniques that are compatible with industrial fabrication processes.

2D materials typically have stable and metastable phases with diverse crystal polymorphs and stoichiometries. Metastable phases have structures distinct from their bulk counterparts, which results in materials with intriguing properties. However, such phases are challenging to prepare and stabilize by standard processes or in useful quantities. In an Article, Lina Liu and colleagues report the direct synthesis of metastable pentagonal PdTe₂, epitaxially grown and stabilized on a Pd(100) surface. Unlike common hexagonal building blocks, pentagonal lattices have low crystallographic symmetry and are predicted to present interesting quantum phenomena such as the room-temperature quantum spin Hall effect and magnetic Dirac fermions. However, few 2D pentagonal materials have been synthesized so far. The Pd(100) surface has a square arrangement of Pd atoms and is commensurate to the 2D pentagonal monolayer, which is ideal for the formation of PdTe₂ by tellurization and for stabilizing the pentagonal lattice. It has been shown that PdTe₂ has an indirect bandgap of 1.05 eV, and the non-trivial properties of this unique 2D polytype warrants further investigation, as discussed by Thomas Heine in an accompanying News & Views article.

Compared with pentagonal 2D materials, 1T’-phase transition metal dichalcogenides are the more widely studied metastable 2D materials. In an Article, Zijian Li and colleagues report they use 1D Au nanowires with the unconventional 4H phase to grow and stabilize high-phase-purity 1T’-transition metal dichalcogenide monolayers on the nanowire surface. The formed core–shell nanowires show high surface-enhanced Raman scattering sensitivity, enabling attomole-level detection limits for rhodamine 6G and various severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike proteins. In an accompanying News & Views article, Yeonhee Lee and colleagues comment that these core–shell structures open avenues for the synthesis of metastable phases and also offer potential for developing sensing platforms based on the hybrid structures.

Helicity introduces an additional degree of freedom to phase engineering of low-dimensional structures, but realization in densely packed inorganic crystals is challenging. In an Article, Dmitri Leo Mesoza Cordova and colleagues report the synthesis of 1D GaSeI van der Waals helical crystals by substituting the smaller Ga atoms into the quasi-tetrahedrally coordinated In sites in InSeI. These results may inspire studies on van der Waals helices that reveal the relationship between helicity and structural aperiodicity in the packing of 1D phases and to create other aperiodic low-dimensional materials with tunable helical parameters.

In situ synthesis and phase engineering are useful strategies for tuning properties on-demand. In an Article, Xiaowei Liu and colleagues propose a generic approach that allows in situ tuning between 2D phases of distinct stoichiometries by using a device configuration compatible with electrical transport measurements. The phase transition is triggered by metal atom diffusion from the electrodes into the channel layer (pictured). Different phases can be achieved by controlling the thickness and spacing of the electrodes. This approach enables the in situ tuning of phases and their associated functions, such as varying contact resistance in transistors and customizing the activity of electrocatalysts. Such on-device phase-engineering strategy can be extended to 29 different metal and chalcogen element combinations. In an accompanying News & Views article, Yongjoon Lee and Heejun Yang highlight the potential of this technique for the fabrication of versatile short-channel devices, low-contact-resistance devices and fine patterning in devices.

As phase-engineering methods advance, it is essential to develop robust theoretical models and experimental tools for real-time monitoring and control of phase changes at the nanoscale. Ultimately, the goal is to implement these techniques for real-world applications, once challenges related to scalability, compatibility and sustainability are overcome.