Recently，Prof. LIU Lei's group from Peking University，the user of the Steady-state High Magnetic Field Experimental Facility (SHMFF), Hefei Institutes of Physical Science (HFIPS) of Chinese Academy of Sciences (CAS), together with Prof. WANG Zhaosheng from High Magnetic Field Center, HFIPS of CAS and other co-authors revealed the structure-property relationship in 2D amorphous materials for the first time by the study of amorphous monolayer carbon (AMC).
The relevant research was published in Nature.
The paradigm of "microstructure determines properties" has been spectacularly successful in explaining and predicting behaviors of crystalline materials, and purposely manipulating materials' properties. In amorphous materials, the arrangement of atoms has no long-range order and the internal atoms cannot be directly observed, leading to the puzzle of its three-dimensional atomic structure, and the relationship between atomic scale structure and performance is still unclear. Exploring and characterizing the disorder in amorphous structures is a long-standing riddle in materials science and condensed matter physics.
To address this problem, the research teams used the feature that "the atoms of two-dimensional materials are all exposed on the surface, and their positions can be precisely analyzed" to resolve the atomic structure of amorphous materials.
They used a cyclic aromatic molecule as the precursor and adopted the chemical vapor deposition method. The temperature of the metal substrate was selected as the main control parameter to precisely control the degree of thermal cracking of the precursor and the growth of AMC with different degree of disorder (DOD). Further, electron diffraction and scanning transmission electron microscopy techniques were used to reveal the atomic structure of AMC, and the temperature-dependent characteristics of medium-range order (MRO) differences and atomic structure in AMC were systematically analyzed.
In the electrical measurement of AMC, researchers discovered a highly temperature-dependent characteristics: at low temperatures (275-300 ℃), AMC exhibits high conductivity because of the weak MRO in AMC-275/300, while samples obtained at 325 ℃ become insulating. Furthermore, Rs is negatively correlated with growth temperature at higher temperatures. Researchers have ultimately achieved continuous adjustability of AMC conductivity in nine orders of magnitude. Variable-temperature Hall measurements with SHMFF provided important evidence for determining the electrical conductance behavior of AMC samples.
Researchers have successfully correlated the atomic structure and electrical properties of two-dimensional amorphous carbon using density functional theory calculations and Monte Carlo simulations, revealing the micro-mechanisms of AMC's conductivity differences. They introduced a new structural order parameter -the average density of conducting sites and successfully plotted the "microstructure-macroscopic electrical performance" phase diagram by combining middle range order. This finding also demonstrates the complexity of DOD in amorphous materials, which cannot be described by MRO alone.
This work represents the first precise structure-property relationship in an amorphous material, providing new ideas for the fields of two-dimensional materials, amorphous materials physics and applications.
Figure 1a: Schematic diagram of AMC growth mechanism; b-h: Atomic-scale structural characterizations of AMC (Image by LIU Lei)
Figure 2.a-f: Electrical characterizations of AMC (Image by LIU Lei)
Figure 3.a-h: Theoretical analysis of the relationship of DOD and conductivity in AMC (Image by LIU Lei)