Temporal Evolution of Surface Contamination under Ultra-high Vacuum

Abstract

Ultra-high vacuum (UHV) is essential to many surface characterization techniques and is often applied with the intention of reducing exposure to airborne contaminants. Surface contamination under UHV is not well-understood, however, and introduces uncertainty in surface elemental characterization or hinders surface-sensitive manufacturing approaches. In this work, we investigated the time-dependent surface composition of gold samples with different initial levels of contamination under UHV over a period of 24 h with both experiments and physical modeling. Our results show that surface hydrocarbon concentration under UHV can be explained by molecular adsorption–desorption competition theory. Gold surfaces that were initially pristine adsorbed hydrocarbons over time under UHV; conversely, surfaces that were initially heavily contaminated desorbed hydrocarbons over time. During both adsorption and desorption, the concentration of contaminants tended toward the same equilibrium value. This study provides a comprehensive evaluation of the temporal evolution of surface contamination under UHV and highlights routes to mitigate surface contamination effects.

Mitigating Contamination with Nanostructure-Enabled Ultraclean Storage

“…To demonstrate that these differences in CA relate closely to the surface hydrocarbon concentration, we performed XPS experiments on samples that were exposed to air and samples that were stored in the Hierarchical container after 100 h (Figure A,B). The difference of atomic carbon percentages between these two samples obtained from the XPS spectra is over 5% (the actual difference could be even greater based on our previous work), suggesting a positive correlation between the advancing CA obtained here and the state of surface cleanliness. The experimental results for samples precontaminated in the hexane-rich glass container displayed similar trends (Supporting Information Section 6 and Figure S5).…”

The Surface Energy of Hydrogenated and Fluorinated Graphene

“…In the case of VOC adsorption, the nonadditive nature of dispersive interactions makes it challenging to identify the cause of relatively small changes in dispersive surface energy, especially considering the speed of adsorption and the chemical similarity between graphene and VOCs . Despite our best efforts, preventing VOC adsorption is practically impossible, even under vacuum conditions . Thus, a few layers of VOCs can quickly adsorb and alter the dispersive interaction between graphene and our probe liquid.…”

“…33 Despite our best efforts, preventing VOC adsorption is practically impossible, even under vacuum conditions. 37 Thus, a few layers of VOCs can quickly adsorb and alter the dispersive interaction between graphene and our probe liquid. This is evident in Figure 3a,b, where the diiodomethane contact angle, and corresponding dispersive surface energy, changes little with time, potentially indicating that the determined dispersive surface energy is being driven by VOC adsorption.…”

“…First, the surface energy of supported graphene is not a fixed value as it is affected by its supporting substrate, leading to so-called wetting translucency. − Second, transferring graphene from its growth substrate to a target substrate typically involves several processing steps that can contaminate its surface with various ions and residues, affecting the intrinsic surface interactions. Finally, the adsorption of volatile organic compounds onto a surface in ambient conditions is a confounding factor across all of interfacial science. − Volatile organic compounds (VOC) are compounds that have a high vapor pressure and low water solubility. These VOC are emitted from a wide range of human-made chemicals and can adsorb to surfaces and screen intrinsic surface interactions .…”

Analyzing Properties of Monolayer MoS2 Using RHEED and Ultrafast Electron Diffraction

“…18). The system was then pumped down to an ultra-high vacuum (2 x 10 -10 torr) via a series of scroll, roughing, turbo, and ion pumps to reduce surface deterioration over time 26 .…”