Imaging mass spectrometry on the nanoscale with cluster ion beams

Abstract

Imaging with cluster secondary ion mass spectrometry (SIMS) is reaching a mature level of development. Using a variety of molecular ion projectiles to stimulate desorption, 3-dimensional imaging with the selectivity of mass spectrometry can now be achieved with submicrometer spatial resolution and <10 nm depth resolution. In this Perspective, stock is taken regarding what it will require to routinely achieve these remarkable properties. Issues include the chemical nature of the projectile, topography formation, differential erosion rates, and perhaps most importantly, ionization efficiency. Shortcomings of existing instrumentation are also noted. Speculation about how to successfully resolve these issues is a key part of the discussion.

Figure 1

AFM images of a typical C₆₀⁺ depth profile into a film of trehalose deposited onto silicon. The crater is approximately 340 μm square and 220 nm deep. Note the small amount of roughness at the crater bottom. (a) top-down view; (b) cross section view. Reprinted from ref (21). Copyright 2006 American Chemical Society.

Figure 2

Computer simulations of the sputtering of a silicon target by 10 keV C₆₀⁺ after 300 individual impacts. The topography is shown in (a) with yellow and orange indicating material above the surface, while purple and blue are atoms in craters below the surface. The evolution of roughness with impacts is shown in (b). Reprinted with permission from ref (22). Copyright 2013 John Wiley & Sons, Inc.

Figure 3

[AlQ₃ + H]⁺ intensity versus depth into the structure, whose schematic is shown on the right. Alq₃ is purple, and NPB is orange. The Alq₃ layers are 3 nm thick. Reprinted with permsission from ref (25). Copyright 2014 John Wiley & Sons, Inc.

Figure 4

z-corrected images of BrdU localized within HeLa cells. The image is 202 × 202 μm² and contains 24 slices. The BrdU^– signal is shown in blue, and the sum of C_xH_yO_z^– fragment ions are shown in red. Reprinted from ref (9b). Copyright 2013 American Chemical Society.

Figure 5

C₆₀-SIMS images of a tetrahymena single cell with implanted TiO₂ nanoparticles. The image in (a) is the phosphatidylcholine signal (m/z 184.1) on the surface of a frozen hydrated cell, using the x–y plane. The image in (b) shows the m/z 184.1 signal after etching through the cell surface. The corresponding TiO⁺ signals are shown in panels (c) and (d). To show the effect of differential sputtering rates, x–z images are plotted in (e) and (f). The (e) image shows the Si substrate, and the (f) panel shows the distorted Ti particle, which sputters much more slowly than the cellular material. The field of view here is 70 × 70 μm². Reprinted with permission from ref (28). Copyright 2014 John Wiley & Sons, Inc.