Recently, Weigao Xu’s group of the School of Chemistry and Chemical Engineering, in collaboration with Zehua Hu’s group of the School of Electronic Science and Engineering, and Rui Su’s group at Nanyang Technological University, developed a 2N method to construct artificial three-dimensional (3D) crystals and explored designable excitonic effects in artificial crystals. The results have been published on March 19, 2025 in Nature Communications entitled “Designable excitonic effects in van der Waals artificial crystals with exponentially growing thickness”.
When transition metal dichalcogenides (TMDCs) are thinned down from bulk to monolayers, they exhibit strong excitonic effects and exotic optical properties.Nevertheless, the absolute excitonic responses are quite low, arising from their thin three-atomic-layer-thick structure.Reassembling 2D TMDC layers to build bulk excitonic crystals can significantly boost their optical performance and introduce emerging functionalities toward optoelectronic and valleytronic applications. However, maintaining or manipulating 2D excitonic properties to achieve performance enhancement in bulk artificial crystals is challenging.
Figure 1:A 2N method for fabricating artificial 3D crystals.
To address this, the team developed a 2N approach (Figure 1) for constructing m∙2N-layer artificial excitonic crystals by only a number N of stacking operations, where m denotes the layer number of the initial functional units and their coupled structures. This approach offers three unique advantages: 1) significantly reduces the number of stacking operations required for the bottom-up construction of artificial 3D crystals, enabling the exponentially increasing layer number; 2) allows for the accurate control and replication of interlayer coupling states with 0° twist angles; 3) provides a high degree of freedom.
Figure 2: Millimeter-scale 16-layer MoS2 artificial crystals with monolayer-like excitonic properties.
Starting from centimeter-scale monolayer MoS2 single crystals, a millimeter-scale AA-stacked 16-layer MoS2 can be fabricated through only four stacking operations, which retains monolayer-like exciton properties and exhibits remarkable enhancements up to 643% and 646% in their absorption and photoluminescence (PL) features compared to monolayers, respectively. Weak interlayer coupling is the key factor that allows the preservation of monolayer-like excitonic properties in the crystal (Figure 2).
Figure 3: An 8-layer WSe2/(MoS2/WSe2)3/MoS2 superlattice with enhanced quadrupolar interlayer exciton emission.
Starting from WSe2/MoS2 superlattices with near-infrared II region dipolar interlayer exciton (DIX) emission, an 8L WSe2/(MoS2/WSe2)3/MoS2 superlattice with highly consistent interlayer coupling states can be fabricated through only two stacking operations, which demonstrated an intensity increase of up to 400% of quadrupolar interlayer exciton (QIX) emission as compared to DIX emission in its bilayer counterpart. Benefiting from the high-precision replication of interlayer coupling states, the team constructed superlattices with 2L, 3L (e⁻-h⁺-e⁻, and h⁺-e⁻-h⁺ types), and 4L regions and first observed a careful investigation of the evolution of QIXs and DIXs (Figure 3).
Our work shows a promising approach for the design and bottom-up fabrication of excitonic crystals, promoting the exploration of excitonic physics in complex van der Waals structures and their applications in optoelectronic devices. Benefiting from the great success in synthesizing different types of wafer-scale 2D single crystals, as well as progress in the robotic assembly technique, the 2N method offers the potential of scalable and rapid production of freely designed macroscale structures with exotic properties.
Ph.D. candidate Qianlu Sun, Ph.D. candidate Jiamin Lin at Nanjing University, Ph.D. Pedro Ludwig Hernandez-Martinez at Nanyang Technological University and Prof. Taotao Li at Nanjing University are co-first authors of the paper. Prof. Weigao Xu, Prof. Hu Zehua at Nanjing University, and Prof. Su Rui at Nanyang Technological University are co-corresponding authors. The research groups of Taotao Li, Peng Wang, Nannan Mao, Changjin Wan at Nanjing University and Huakang Yu at South China University of Technology, have provided supports in the large-area growth of MoS2 single crystals, aberration-corrected scanning transmission electron microscopy characterization, data analysis, and theoretical calculations, respectively. This work was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, and the Natural Science Foundation of Jiangsu Province.
Link to article: https://www.nature.com/articles/s41467-025-57759-w