Probing liquid surfaces under vacuum using SEM and ToF-SIMS

Abstract

We report a newly developed self-contained interface for high-vapor pressure liquid surfaces to vacuum-based analytical instruments. It requires no wires or tubing connections to the outside of the instrument and uses a microfluidic channel with a 3 μm diameter window into the flowing fluid beneath it. This window supports the liquid against the vacuum by the liquid's surface tension and limits the high-density vapor region traversed by the probe beams to only a few microns. We demonstrate this microfluidic interface for in situ liquid surfaces in a time-of-flight secondary ion mass spectrometer (ToF-SIMS) and a scanning electron microscope (SEM) with chemical analysis.

Improving the Molecular Ion Signal Intensity for In Situ Liquid SIMS Analysis

“…This condition has been successfully used in our recent biofilm and single cell research [21,27]. Figure 5 compares the positive and negative ion spectra of a living biofilm sample using a 200 nm 0.20 pA Bi + beam (a typical condition used in our initial papers [11][12][13][14][15][16][17]) and a 500 nm 0.36 pA + beam appeared similar to the negative secondary ion spectrum using the 200 nm 0.20 pA Bi + beam. However, the signal intensity of the former also showed a~two orders of magnitude improvement.…”

“…In most of our previous studies, a 25 keV Bi + beam was used as the primary ion beam, and the beam size was focused tõ 200 nm diameter with a current of 0.2-1.0 pA [11,[13][14][15]. The Bi + beam was chosen because it was the most abundant primary ion species, and it could provide the highest current with our instrument (IONTOF V) [29,30].…”

“…The details of SALVI fabrication have been extensively described in our previous publications [11,12,16]. In brief, a microfluidic channel with a cross-section of 200 × 300 μm 2 was prepared on a PDMS block.…”

“…However, initial studies reported detection of only negative molecular ions [11][12][13][14][15][16][17] because the signals of the positive ion spectra were too weak to be meaningful. In addition, although negative molecular ions could be successfully analyzed, their signal intensity was not strong [16,17].…”

“…In situ liquid SIMS was developed in the last several years and one of its unique capabilities is the analysis of liquid-solid and liquid-vacuum interfaces [11][12][13][14][15][16][17][18]. To enable in situ liquid SIMS measurement, a vacuum-compatible device named system for analysis at the liquid-vacuum interface (SALVI) was developed [11,12], where a liquid of interest can be sealed in a microfluidic channel under a thin (e.g., 100 nm) silicon nitride (SiN) membrane. The key to in situ liquid SIMS measurements is the use of a~2 μm diameter aperture on the thin SiN membrane, through which the liquid interface and the liquid itself can be analyzed.…”