Understanding the Underlying Field Evaporation Mechanism of Pure Water Tips in High Electrical Fields

Abstract

We investigated the field evaporation process of frozen water in atom probe tomography (APT) by density functional simulations. In previous experiments, a strong tailing effect was observed for peaks caused by the molecular structure (H 2 O) n H + , in contrast to other peaks. In purely field-induced and thermally assisted evaporation simulations, we found that chains of protonated water molecules were pulled out of the dielectric surface by up to 6 Å, which are stable over a wide range of field strengths. Therefore, the resulting water clusters experience only part of the acceleration after evaporation compared to molecules evaporating directly from the surface and, thus, exhibit an energy deficit, which explains the tailing effect. Our simulations provide new insight into the complex evaporation behavior of water in high electrical fields and reveal possibilities for adapting the existing reconstruction algorithms.

Nanoscale analysis of frozen honey by atom probe tomography

“…Water has been the primary component in the case of the earlier measured glucose solution. The ionization field strength of water (2–10 V/nm) 28 , 29 , 53 is rather low in comparison to conventional metallic samples. In contrast, the main component of honey is made of monosaccharides (glucose, fructose) and higher bonded sugars, while only a minority fraction of 12–22 at.% is water.…”

“…In general, the mechanism of field evaporation for organic species is more complex than for metals, leading to molecular fragments of C x H y /C x O y H z groups instead of single atoms. In our various APT experiments with soft matter and liquids and related DFT-based calculations 20 , 21 , 43 , 44 , 53 , we gathered increasing evidence that the field strength during the continuous field desorption is the decisive factor which in turn is mostly affected by the bonding strength of the matrix materials. For example, alkane chains strongly bonded by a thiol group to a metallic substrate evaporate in rather small hydrocarbons with one or two carbon atoms.…”

“…Peak identification becomes ambiguous due to many possible combinations of charge states and chemical species. The lowest possible charge states are assumed in assigning a molecular identity, since higher charge states are unlikely due to the low observed evaporation field of water or glucose solution 28 , 29 , 53 . The likely possibilities for peak assignment are listed in Table S2 (see Supplementary Material).…”

“…SEM images were used to determine the initial tip radius and taper angle. The tip radius and taper angle were used to optimize the calculated evaporation field curve to be in the range of the field strength of water which is about 8–12 V/nm 53 . All necessary parameters, such as field and image compression factors were determined in earlier measurements.…”

Nanopore ion sources deliver single amino acid and peptide ions directly into high vacuum

“…Atom probe tomography, a technique for generating compositional maps of materials using field-emitted ions, is entering a new frontier with studies of liquid samples [63][64][65], which may be simplified by using a nanopore ion source. Finally, the nanopore source may shed new experimental light on the ionization of water, its orientation, or its evaporation at highly electrified surfaces [66,67].…”