⇦Chinese Physics Letters, 2023, Vol. 40, No. 5, Article code 056101Viewpoint When 2D Materials Encounter Disorder Lei Liu (刘磊)* Affiliations School of Materials Science and Engineering, Peking University, Beijing 100871, China Received 17 March 2023; accepted manuscript online 13 April 2023; published online 27 April 2023 *Corresponding author. Email: l_liu@pku.edu.cn Citation Text: Liu L 2023 Chin. Phys. Lett. 40 056101    Abstract
cpl-40-5-056101-fig1.png
 DOI:10.1088/0256-307X/40/5/056101 © 2023 Chinese Physics Society Article Text Back to the 1950s, Anderson revealed the absence of wave diffusion, i.e., Anderson localization, in a sufficiently disordered media, marking the start of electron transport research in non-periodic lattices.[1] In a simplified theoretical model, one critical order parameter was proposed to determine the boundary of electron delocalization, even though one did not know the atomic structures in a realistic disordered system at that moment.[1] Even up to date, the full understanding of precise atomic configurations, including the variation of bonding lengths/angles and local stacking, of a three-dimensional (3D) vitreous solid is extremely challenging,[2] due to the “unexposed” and disordered features of inner atoms. On a parallel front, with a reduced dimension, that is, from 3D to two dimensions (2D), all atoms are arranged on the surface, facilitating the direct observation of atoms by the state-of-the-art electron microscope in crystalline and amorphous materials, such as the single-layered silica and carbon.[3-5] While scientists have already uncovered the secret of atom structures in 2D amorphous phases,[4,5] a causal link between the degree of disorder (DOD) and macroscopic properties remains elusive.[6]
cpl-40-5-056101-fig1.png
Fig. 1. Atomic schematic of electron hopping between 6-membered-ring nanocrystallites separated by non-six-membered regions in amorphous monolayer carbon (AMC). The hopping efficiency is highly related to the average density of conducting sites that was proposed as the second order parameter to fully describe the DOD of AMC.[6]
The paper by Tian et al.[6] provides one real case to build the structure-property relationship in one amorphous material at the fundamental level. Using heteroarene molecules as precursors to grow AMC by the thermal chemical vapor deposition (CVD) method, the temperature is founded as a practical and critical parameter to continuously tune DOD and 2D variable-range-hopping electron conduction in the range of nine orders of magnitude, achieving a conductor–insulator transition (Fig. 1). Atomic images of electron microscope directly show the highly distorted nanocrystalline regions embedded in the continuous random network matrix and the temperature-dependence absence/presence of medium-range order (MRO) in AMC. Moreover, two order parameters, the degree of MRO and the average density of conducting sites, are proposed for the first time to fully describe DOD. Based on theoretical calculations, the link between atomic structures and electrical conductivity is built, giving rise to the complete “DOD-sheet resistance” diagram, which agrees with experimental findings. Following the paradigm of “Seeing Is Believing”, Tian et al. answered one fundamental question for amorphous materials, although in a simplified dimension, experimentally echoing Anderson's models with exact atom positions after more than half a century.[6,7] Other types of amorphous monolayers, if the DOD could be controlled well, are of high interest as well to see whether the Anderson model can be applicable or not.[4,8] Unlike the bulk synthetic amorphous materials which typically undergo an ultrafast cooling process from liquid states,[9] the bottom-up CVD growth of amorphous monolayers on widely used metal substrates bears a high level of resemblance to the growth of crystalline, atomically thin films except for lower synthesis temperatures.[10] Combining nanofabrication techniques with sequential growths,[11] in-plane crystalline-amorphous hetero-/homo-junctions are feasible in the one-atom-thick limit. Moreover, the method may be general and applicable for all 2D materials, thus establishing a large group of amorphous structures, while the whole idea boils down to a question whether one can stabilize its amorphization in 2D space. From the application point of view, the ultrahigh specific surface area, the plenty of sites with dangling bonds, and the DOD-tuned electrical properties endow amorphous monolayers with functionalities and potentials in catalysis, energy, and sensing.[8] The investigations of AMC may usher in an ear of amorphous monolayer materials with exceptional prosperity and opportunities. References Absence of Diffusion in Certain Random LatticesDetermining the three-dimensional atomic structure of an amorphous solidDirect Determination of the Chemical Bonding of Individual Impurities in GrapheneImaging Atomic Rearrangements in Two-Dimensional Silica Glass: Watching Silica’s DanceSynthesis and properties of free-standing monolayer amorphous carbonDisorder-tuned conductivity in amorphous monolayer carbonConstrained Online Two-stage Stochastic Optimization: New Algorithms via Adversarial LearningAmorphizing noble metal chalcogenide catalysts at the single-layer limit towards hydrogen productionNon-crystalline Structure in Solidified Gold–Silicon AlloysUnusual role of epilayer–substrate interactions in determining orientational relations in van der Waals epitaxyHeteroepitaxial Growth of Two-Dimensional Hexagonal Boron Nitride Templated by Graphene Edges
[1] Anderson P W 1958 Phys. Rev. 109 1492
[2] Yang Y et al. 2021 Nature 592 60
[3] Zhou W et al. 2012 Phys. Rev. Lett. 109 206803
[4] Huang P Y et al. 2013 Science 342 224
[5] Toh C T et al. 2020 Nature 577 199
[6] Tian H F et al. 2023 Nature 615 56
[7] Cheng M et al. 2023 arXiv:2302.00997 [cs.LG]
[8] He Y M et al. 2022 Nat. Catal. 5 212
[9] Klement W, Willens R H, and Duwez P 1960 Nature 187 869
[10] Liu L et al. 2014 Proc. Natl. Acad. Sci. USA 111 16670
[11] Liu L et al. 2014 Science 343 163