Synthesis and properties of free-standing monolayer amorphous carbon (2105.08926v1)

Abstract: Bulk amorphous materials have been studied extensively and are widely used, yet their atomic arrangement remains an open issue. Although they are generally believed to be Zachariasen continuous random networks, recent experimental evidence favours the competing crystallite model in the case of amorphous silicon. In two-dimensional materials, however, the corresponding questions remain unanswered. Here we report the synthesis, by laser-assisted chemical vapour deposition, of centimetre-scale, free-standing, continuous and stable monolayer amorphous carbon, topologically distinct from disordered graphene. Unlike in bulk materials, the structure of monolayer amorphous carbon can be determined by atomic-resolution imaging. Extensive characterization by Raman and X-ray spectroscopy and transmission electron microscopy reveals the complete absence of long-range periodicity and a threefold-coordinated structure with a wide distribution of bond lengths, bond angles, and five-, six-, seven- and eight-member rings. The ring distribution is not a Zachariasen continuous random network, but resembles the competing (nano)crystallite model. We construct a corresponding model that enables density-functional-theory calculations of the properties of monolayer amorphous carbon, in accordance with observations. Direct measurements confirm that it is insulating, with resistivity values similar to those of boron nitride grown by chemical vapour deposition. Free-standing monolayer amorphous carbon is surprisingly stable and deforms to a high breaking strength, without crack propagation from the point of fracture. The excellent physical properties of this stable, free-standing monolayer amorphous carbon could prove useful for permeation and diffusion barriers in applications such as magnetic recording devices and flexible electronics.

• The paper demonstrates a novel laser-assisted CVD process that synthesizes uniform, centimeter-scale MAC at low temperatures.
• The study reveals that MAC features a heterogeneous atomic topology and high mechanical resilience, with a breaking strength of 22 Nm⁻¹.
• DFT and quantum simulations confirm MAC's stability and unique high-resistivity behavior, highlighting its potential in electronic and barrier applications.

Synthesis and Properties of Free-Standing Monolayer Amorphous Carbon

The paper offers a comprehensive paper on the synthesis and distinct properties of monolayer amorphous carbon (MAC), a material synthesized via a laser-assisted chemical vapor deposition (CVD) method. This research tackles prevailing ambiguities surrounding the atomic-scale structure of amorphous materials, which remain inconclusive, particularly in two-dimensional (2D) materials contexts. Traditionally, amorphous materials have been theorized as continuous random networks (Z-CRNs) or as composed of embedded nanocrystallites. This research focused on the latter model as being more consistent with the observed properties of MAC.

The authors describe a self-limiting CVD process that yields uniform, centimeter-scale MAC monolayers at relatively low substrate temperatures (200-250°C). These MAC samples are characterized using advanced techniques such as atomic-resolution transmission electron microscopy (TEM) and Raman spectroscopy, thereby revealing a lack of long-range periodicity. The atomic imaging shows MAC to possess a heterogeneous topology with 5-, 6-, 7-, and 8-member rings, contrasting with the periodic crystalline arrangement in graphene. The analysis confirms a broad distribution of bond lengths and angles, indicating a non-random close packing distinct from both graphene and expected Z-CRN configurations.

One of the prominent findings of the research is MAC's mechanical resilience. High-resolution TEM characterizations demonstrate that MAC is not only stable but also possesses a high breaking strength, and deformation does not engender crack propagation. Mechanically, MAC shows enhanced elasticity and strength parameters, exhibiting a breaking strength of 22. Nm⁻¹, which is more than half that of monolayer crystalline graphene. The unique mechanical properties are attributed to the non-hexagon members and distortions, which contribute to the material's amorphous structure.

Electrically, MAC manifests a high resistivity, mimicking behavior typical of disordered quasi-1D systems rather than the expected 2D Mott variable range hopping. This is evidenced by substantial resistivity values (~100 GΩ) and unique I-V characteristics, suggesting potential utility in applications where high electrical resistance is beneficial, similar to those using boron nitride.

Density-functional-theory (DFT) calculations corroborate these findings, showing structural conformity between the experimentally observed and theoretically modeled MAC. Quantum molecular dynamics simulations further illustrate that the structure maintains stability at ambient conditions, affirming MAC as a potential candidate for technological applications such as selective ion transport membranes, electronic insulators, and possibly even photonic devices considering its observed photoluminescence and broad optical band gap of approximately 2.1 eV.

The implications of these findings lie in the demonstrated method for synthesizing and harnessing the unique properties of MAC, which could advance applications in electronic, mechanical, and barrier materials. Future explorations could dive deeper into tailoring the electronic landscape by patterning the amorphous network to target specific technological applications. This research paves the way for exploring other 2D amorphous structures and their potential utilities, contributing to a deeper understanding of non-crystalline materials in low-dimensional regimes.