Revealing the innate sub-nanometer porous structure of carbon nanomembranes with molecular dynamics simulations and highly charged ion spectroscopy

TL;DR

The paper addresses the challenge of characterizing the atomistic structure of carbon nanomembranes (CNMs) under radiation-sensitive conditions. It advances the field by coupling two MD modeling strategies (exclusion-cylinder MD and momentum-transfer simulations) with time-dependent potential HCI transmission spectroscopy to interpret angle- and charge-exchange data, revealing a sub-nanometer porous CNM with substantial under-coordination. The best-fit structure, a 150-cylinder membrane annealed for 9 ps, reconciles the observed high-charge-state ion transmission with tensile-modulus data in the experimental range and implies ambient reactivity stabilized by passivation. This work provides a quantitative, mechanism-informed atomistic description of CNMs, linking structural porosity and bonding topology to mechanical and chemical properties and offering guidance for CNM design and processing in practical environments.

Abstract

Carbon nanomembranes (CNMs) are nanometer-thin disordered carbon materials that are suitable for a range of applications, from energy generation and storage, through to water filtration. The structure-property relationships of these nanomembranes are challenging to study using traditional experimental characterization techniques, primarily due to the radiation-sensitivity of the free-standing membrane. Highly charged ion spectroscopy is a novel characterization method that is able to infer structural details of the carbon nanomembrane without concern of induced damage affecting the measurements. Here we employ molecular dynamics simulations to produce candidate structural models of terphenylthiol-based CNMs with varying degrees of nanoscale porosity, and compare predicted ion charge exchange data and tensile moduli to experiment. The results suggest that the in-vacuum CNM composition likely comprises a significant fraction of under-coordinated carbon, with an open sub-nanometer porous structure. Such a carbon network would be reactive in atmosphere and would be presumably stabilized by hydrogen and oxygen groups under atmospheric conditions.

Figures (7)

• Figure 1: Exit charge state spectra of 90keV Xe$^{10+}$ after transmission through the CNM before (a) and after (b) contaminant removal through ion irradiation. (c) A fluence of $\sim 3\times 10^{11}$ ion impacts per cm2 is necessary to reach an equilibrium mean charge state of $q\textsubscript{out}\sim 2$.
• Figure 2: Schematic diagram of the specular reflection imposed on atoms that enter the exclusion cylinders during region restricted dynamics.
• Figure 3: Planar top-down view of a selection of exclusion cylinder enforced carbon nanomembrane structures with atoms colored according to their coordination: sp$=$ blue, sp$^2=$ orange, sp$^3=$ purple, and single coordinated are shown in green
• Figure 4: Predicted average tensile moduli of the CNM models as a function of the number of exclusion cylinders (panel a), where the color of each point represents the annealing time each structure has been subjected to. Each data point has been averaged over both in-plane directions, and error bars denote one standard deviation from the mean. The shaded green region indicates the reported experimental tensile modulusZhang2011 range. Panel b) presents the same tensile data as a function of sp$^2$/sp for 1ps, 16ps, and 64ps simulated annealed structures, where the enforced cylinder counts are indicated by the different markers and colors.
• Figure 5: Simulated exit charge state for Xe ions incident on various hole-enforced nanomembrane structures. Structures that have been annealed for 0, 16, and 64ps are shown in darkening blue shades, and the 9ps structure is shown in orange.