Two-dimensional (2D) materials have captivated the attention of the scientific community (Chem. Soc. Rev. 2022, 51, 7732–7751), showcasing a breadth of exceptional properties and finding applications in a myriad of fields (Nature 2023, 616, 470–475). Over the past decades, these materials have revealed themselves in two predominant forms: layered and non-layered, distinguished by the nature of their interlayer interactions (Nat. Commun. 2020, 11, 3979). These atomically thin layers are held together either by van der Waals (vdW) forces or covalent bonds, with the interactions occurring perpendicular to the bi-dimensional growth plane of the material. This results in a unique and orthogonal structural configuration, primarily driven by the atoms’ inclination to adopt a closely packed arrangement, minimizing entropy and fortifying structural stability (Chem. Soc. Rev. 2022, 51, 7327–7343). This robust configuration poses a substantial challenge to uncovering the intricate structures and interactions beneath.
Fig. 1| Interactive-strength-modulation growth strategy and stability of 2D quasi-layered domino-structured gallium telluride (QLDS-GaTe) single crystal. a, Depiction of the interaction and growth orientation of layered GaTe on van der Waals (vdW) substate. b, Illustration of the interaction and growth orientation of QLDS-GaTe on the strong interactive substrate. c, Scheme of skewed angle-related interaction strength perpendicular to the 2D growth plane. d, Calculated contour maps of charge density difference for 2D QLDS-GaTe structure. Electron accumulation (depletion) is shown in red (blue), in accordance with the positive (negative) value below. Yellow atoms refer to Te atoms, and blue atoms refer to Ga atoms. e, Calculated phonon dispersive curves for 2D QLDS-GaTe structure at a temperature of 300 K. f, Temperature and total energy trajectories as derived from molecular dynamics simulations of the 2D QLDS-GaTe structure, conducted at 300 K.
In our recent exploration, Prof. Fu in Wuhan University guided us to delve into a unique class of materials, identifying special interlayer interactions that comprise a synergistic blend of vdW forces and covalent bonds. Notably, these forces are not perpendicular to the 2D growth plane, resembling a series of domino tiles (Fig. 1). This interaction results in a force stronger than that found in layered materials, yet weaker than that in non-layered materials. We have termed this structural configuration as a 2D quasi-layered material with a domino structure, highlighting its unique position in the spectrum of 2D materials.
Fig. 2 | Structural characterization of the 2D quasi-layered domino-structured gallium telluride (QLDS-GaTe) single crystal. a, Schematic depiction of the alterations in the lattice constant along the stacking direction following the transition to two-dimensionality in QLDS-GaTe. b, Computed correlation of lattice constant in the stacking direction with respect to thickness. c, Fast Fourier Transform (FFT) pattern derived from a relatively thinner sample of 2D QLDS-GaTe single crystal. d, e, Experimental and corresponding simulated high-resolution transmission electron microscopy (HRTEM) images of the thinner 2D QLDS-GaTe single crystal. f, FFT pattern derived from a comparatively thicker sample of 2D QLDS-GaTe single crystal. g, h, Experimental and corresponding simulated HRTEM images of the thicker 2D QLDS-GaTe single crystal. i, Low-magnification scanning transmission electron microscopy (STEM) image accompanied by corresponding EDS elemental mapping of Ga, Te, and the overlay of Ga and Te elements, observing from the planar perspective.
Materials with 2D quasi-layered domino structures exhibit distinct properties, setting them apart from both layered and non-layered counterparts. Representatively, we have proposed a 2D quasi-layered domino-structured gallium telluride (QLDS-GaTe), showcasing a skewed growth structure and deviating from the substrate orientation by approximately 25° (Fig. 2). This fascinating conformation is accompanied by a pronounced enhancement in interlayer coupling, with the lattice constant contraction along this unique orientation reaching up to 7.7%, closely approaching the theoretical prediction of 10.8%.
The material demonstrated remarkable anisotropy and a pronounced second harmonic generation (SHG) enhancement effect, with its polarization tensor reaching an impressive 394.3 pm V−1 (Fig. 3). These findings opened up new avenues for future explorations in nonlinear optics and other related fields.
Fig. 3 | Investigation of second harmonic generation (SHG in 2D QLDS-GaTe single crystal. a, Schematic representation of the SHG investigation conducted on the 2D QLDS-GaTe single crystal. Blue arrow refers to the incoming light with a frequency of ω, and yellow arrow refers to the second harmonic generated with a frequency of 2ω. Yellow atoms refer to Te atoms, and blue atoms refer to Ga atoms. b, Wavelength-dependent investigation of SHG intensity under excitation power of 1 mW. c, Dependency of SHG intensity on the excitation power. d, Power-dependent SHG intensity plotted on logarithmic coordinates. e, SHG spectra corresponding to each thickness of the sample. f, Thickness-dependent SHG intensity relationship diagram extracted from Fig. 3e.
In conclusion, our innovative concept of "2D quasi-layered materials with domino structures" effectively bridges the gap between traditional layered and non-layered materials, allowing for precise modulation of interlayer interactions and extending applicability across various domains, including optics, sensors, and catalysis. Under the guidance of Prof. Fu and in collaboration with my partner, we at Wuhan University have made significant strides in this domain, contributing to the ever-evolving field of material science.
Last, I, Haihui Lan, owe an immense debt of gratitude to my mentor, Prof. Fu of Wuhan University. From my early days of grappling with the complexities of scientific research, he has been my guiding light. His steadfast belief in my potential and unwavering support have paved the way for my growth, transforming me from a novice to a budding researcher ready to embark on further studies at Massachusetts Institute of Technology. To Prof. Fu, I extend my heartfelt appreciation and deep respect for shaping my academic journey. Your influence will forever resonate in my future endeavors. Thank you.
The details of this work are available from Nature Communications.