Atomically clean free-standing two-dimensional materials through heating in ultra-high vacuum

TL;DR

This study demonstrates that annealing freestanding 2D materials like graphene and hBN at temperatures of 400°C or higher in ultra-high vacuum effectively achieves atomically clean surfaces, enabling better nanoscale engineering and device fabrication.

Contribution

The paper introduces a method for achieving atomically clean 2D materials through high-temperature annealing in ultra-high vacuum, with direct atomically resolved cleanliness assessment.

Findings

Annealing at 200°C reduces contamination significantly.

Over 90% of the material becomes atomically clean at 400°C or higher.

Further cleaning limited by material defects and transfer-related contamination.

Abstract

Surface contamination not only influences but in some cases even dominates the measured properties of two-dimensional materials. Although different cleaning methods are often used for contamination removal, commonly used spectroscopic cleanliness assessment methods can leave the level of achieved cleanliness ambiguous. Despite two decades of research on 2D materials, the true cleanliness of the used samples is often left open to interpretation. In this work, freestanding monolayer graphene and hexagonal boron nitride are annealed at different temperatures in a custom-built ultra-high vacuum heating chamber, connected to a scanning transmission electron microscope via a vacuum transfer line, enabling atomically resolved cleanliness characterization as a function of annealing temperature, while eliminating the introduction of airborne contamination during sample transport. While annealing at 200 {\deg}C already reduces contamination significantly, it is not until 400 {\deg}C or higher, where over 90% of the free-standing monolayer areas are atomically clean. At this point, further contamination removal is mainly limited by defects in the material and metal contamination introduced during the sample transfer or growth. The achieved large, atomically clean areas can then be used for further nanoscale engineering steps or device processing, facilitating interaction with the material rather than contamination.