Amorphous materials, which are characterized by disorder in three-dimensional space, present a unique challenge in analyzing the atomic arrangements and uncover the relationship between structure and properties. 

In a study published in Nature, a group of researchers from University of Chinese Academy of Sciences (UCAS) of the Chinese Academy of Sciences and Peking University (PKU), led by Prof. ZHOU Wu from UCAS and Profs. LIU Lei and CHEN Ji from PKU, has made a breakthrough in understanding the relationship between the structure of amorphous materials and their properties. The study is the first to reveal the relationship between the atomic-scale structure and macroscopic properties in two-dimensional (2D) amorphous monolayer carbon (AMC) materials.  

The researchers first made use of the single-atom structural analysis sensitivity of low-voltage scanning transmission electron microscopy (STEM) to precisely determine the position of every carbon atom in the AMC materials and correlated the degree of structural disorders with conductivity of the materials with the help of theory. This will have implications for the future design and application of amorphous 2D materials.

They then grew the AMC samples using a process called chemical vapor deposition on copper substrates, controlling the degree of structural disorder by manipulating the growth temperature. They found that the conductivity of the AMC materials can be adjusted up to one billion times by changing the growth temperature by just 25 degrees Celsius, highlighting the unique advantages of tuning the degree of disorder in 2D amorphous materials. This level of control over the conductivity of amorphous materials is unprecedented and will have implications for future electronic devices. 

To understand the structure-property relationships of AMC materials, the researchers combined advanced imaging techniques such as real-space atomic-resolution imaging and scanning nano-beam electron diffraction in low-voltage STEM to systematically investigate the structural disorder of a series of AMC materials across different scales.

They found that all AMC samples exhibit significant long-range structural disorder, while different growth temperatures produced samples with distinct degrees of disorder. Notably, the AMC samples grown at 400 °C and 500 °C showed only short-range order, while those grown at 300 °C also showed medium-range order. The differences in the structural disorder of AMC samples grown at different temperatures are reflected in the obvious differences in the proportions and distributions of nanocrystallites and amorphous regions in the samples, which are characterized by honeycomb structures and a high density of 5-7-8 defective rings, respectively. 

Besides, the researchers conducted a systematic theoretical analysis, using both Density Functional Theory (DFT) calculations and Monte Carlo simulations, to establish a correlation between the atomic structure and electrical properties of 2D amorphous carbon. They uncovered the microscopic mechanism underlying the conductivity of AMC materials and established a relationship between the electrical conductivity of AMC materials and two key parameters, namely the level of medium-range order and the average density of conducting sites. 

This study in understanding the relationship between atomic-scale structure and macroscopic properties in 2D amorphous monolayer carbon materials has the potential to advance the utilization of these materials in ultrathin electronic devices and serve as a promising model for investigating other amorphous materials in the future.