[Image above] Restructuring of carbon layers due to the impact of a highly charged ion with initial charges (Qin) ranging from 15–40 for a fixed hopping time of 7 au. The threshold for pore formation at this charge mobility is around Qin = 25. Credit: Sagar Grossek et al., Nano Letters (CC BY 4.0)
By Laurel Sheppard
As discussed on CTT this Tuesday, smaller and smaller materials and devices are being used to create next-generation electronics. To ensure the technology operates properly, researchers need modeling, processing, and measurement methods that work on the nanoscale.
Heterostructures are one area of electronics that benefit from these nanomethods. Heterostructures are a way of designing semiconductors by layering multiple different materials. Such layering allows researchers to control various fundamental parameters in semiconductor devices, such as the bandgap width, the effective masses and mobilities of charge carriers, and the refractive index.
The use of 2D materials to create heterostructures opens the door to fabricating semiconductors with unique electrical, optical, and thermal characteristics. In such heterostructures, each 2D layer is weakly held together by van der Waals forces. As such, electrons can move freely within any given plane but have restricted motion between planes.
To accurately tailor the properties of 2D heterostructures, researchers require modification techniques with single-layer precision. Highly charged ions are one tool that researchers use to manipulate and modify 2D heterostructures with the necessary accuracy.
Highly charged ions are atoms in which all or most of the electrons have been removed. When shot into a 2D heterostructure, these ions act like hand grenades, releasing lots of energy as they rapidly capture electrons from the atoms around them. This reaction results in the formation of nanometer-sized pores in the 2D heterostructure.
Researchers have tested the response of various 2D materials to highly charged ion bombardment, including graphene and molybdenum disulfide. While sometimes the ions penetrate the 2D material without any noticeable change, at other times, the 2D material at the impact site is completely destroyed.
To understand why different materials react so differently to highly charged ion bombardment, researchers from the Institute for Theoretical Physics and the Institute of Applied Physics at TU Wien in Austria used simulations to investigate nanopore formation mechanisms in fluorographene.
Fluorographene is graphene that has been fluorinated, meaning it consists of carbon and fluorine atoms. It is considered one of the thinnest insulators, and the chemistry of fluorographene is a budding discipline because it can lead to various graphene derivatives.
The researchers’ simulations, as described in an open-access paper, consisted of a classical molecular dynamics simulation (for the motion of the target atoms) combined with a Monte Carlo model (for the diffusive charge transport within the layer).
Within the simulations, graphene flakes appeared as several concentric rings around the impact point of the highly charged ion. Atom motion was followed until the lattice was free of charges; in other words, until the charges migrated beyond the outermost ring.
Additionally, the researchers included the Stillinger–Weber potential within their model. This potential, which has been successfully used in describing various 2D materials, states that the bond energy is related to both the distance between atoms and the bond angles. The researchers explain they included this potential “due to its small numerical cost while successfully reproducing the splitting of graphene grains into graphene nanoplatelets when strained.”
Based on these simulations, the researchers determined that it is not the momentum of the highly charged ions that is mainly responsible for pore formation but the 2D material’s charge mobility.
“Graphene has an extremely high electron mobility. So this local positive charge [the highly charged ion] can be balanced there in a short time. Electrons simply flow in from elsewhere,” explains senior author Christoph Lemell, associate professor of theoretical physics, in a TU Wien press release.
In contrast, materials such as molybdenum disulfide have a slower electron mobility, and so electrons cannot flow in from elsewhere fast enough. A mini explosion thus occurs at the impact site: the positively charged atoms from which the ions took their electrons repel each other, and a nanosized pore forms.
This dependence of pore formation on charge mobility explains why highly fluorinated graphene is more susceptible to pore formation—fluorination alters the band structure of graphene, thus reducing charge mobility.
“Through our findings, we now have precise control over the manipulation of materials at the nanoscale,” says lead author Alexander Sagar Grossek in the press release. “This [knowledge] provides a whole new tool for manipulating ultrathin films in a precisely calculable way for the first time.”
The open-access paper, published in Nano Letters, is “Model for nanopore formation in two-dimensional materials by impact of highly charged ions” (DOI: 10.1021/acs.nanolett.2c03894).