Probes, Patterns, and (nano)Particles

philipmoriarty

Philip Moriarty

This is a guest post by Philip Moriarty, Professor of Physics at the University of Nottingham (and blogger).

“We shape our tools, and thereafter our tools shape us.”

Marshall McLuhan (1911-1980)

My previous posts for Raphael’s blog have focussed on critiquing poor methodology and over-enthusiastic data interpretation when it comes to imaging the surface structure of functionalised nanoparticles. This time round, however, I’m in the much happier position of being able to highlight an example of good practice in resolving (sub-)molecular structure where the authors have carefully and systematically used scanning probe microscopy (SPM), alongside image recognition techniques, to determine the molecular termination of Ag nanoparticles.

For those unfamiliar with SPM, the concept underpinning the operation of the technique is relatively straight-forward. (The experimental implementation rather less so…) Unlike a conventional microscope, there are no lenses, no mirrors, indeed, no optics of any sort [1]. Instead, an atomically or molecularly sharp probe is scanned back and forth across a sample surface (which is preferably atomically flat), interacting with the atoms and molecules below. The probe-sample interaction can arise from the formation of a chemical bond between the atom terminating the probe and its counterpart on the sample surface, or an electrostatic or magnetic force, or dispersion (van der Waals) forces, or, as in scanning tunnelling microscopy (STM), the quantum mechanical tunnelling of electrons. Or, as is generally the case, a combination of a variety of those interactions. (And that’s certainly not an exhaustive list.)

Here’s an example of an STM in action, filmed in our lab at Nottingham for Brady Haran’s Sixty Symbols channel a few years back…

Scanning probe microscopy is my first love in research. The technique’s ability to image and manipulate matter at the single atom/molecule level (and now with individual chemical bond precision) is seen by many as representing the ‘genesis’ of nanoscience and nanotechnology back in the early eighties. But with all of that power to probe the nanoscopic, molecular, and quantum regimes come tremendous pitfalls. It is very easy to acquire artefact-ridden images that look convincing to a scientist with little or no SPM experience but that instead arise from a number of common failings in setting up the instrument, from noise sources, or from a hasty or poorly informed choice of imaging parameters. What’s worse is that even relatively seasoned SPM practitioners (including yours truly) can often be fooled. With SPM, it can look like a duck, waddle like a duck, and quack like a duck. But it can too often be a goose…

That’s why I was delighted when Raphael forwarded me a link to “Real-space imaging with pattern recognition of a ligand-protected Ag374 nanocluster at sub-molecular resolution”, a paper published a few months ago by Qin Zhou and colleagues at Xiamen University (China), the Chinese Academy of Science, Dalian (China), the University of Jyväskylä (Finland), and the Southern University of Science and Technology, Guandong (China). The authors have convincingly imaged the structure of the layer of thiol molecules (specifically, tert-butyl benzene thiol) terminating 5 nm diameter silver nanoparticles.

What distinguishes this work from the stripy nanoparticle oeuvre that has been discussed and dissected at length here at Raphael’s blog (and elsewhere) is the degree of care taken by the authors and, importantly, their focus on image reproducibility. Instead of using offline zooms to “post hoc” select individual particles for analysis (a significant issue with the ‘stripy’ nanoparticle work), Zhou et al. have zoomed in on individual particles in real time and have made certain that the features they see are stable and reproducible from image to image. The images below are taken from the supplementary information for their paper and shows the same nanoparticle imaged four times over, with negligible changes in the sub-particle structure from image to image.

This is SPM 101

This is SPM 101. Actually, it’s Experimental Science 101. If features are not repeatable — or, worse, disappear when a number of consecutive images/spectra are averaged – then we should not make inflated claims (or, indeed, any claims at all) on the basis of a single measurement. Moreover, the data are free of the type of feedback artefacts that plagued the ‘classic’ stripy nanoparticle images and Zhou et al. have worked hard to ensure that the influence of the tip was kept to a minimum.

Given the complexity of the tip-sample interactions, however, I don’t quite share the authors’ confidence in the Tersoff-Hamann approach they use for STM image simulation [2]. I’m also not entirely convinced by their comparison with images of isolated molecular adsorption on single crystal (i.e. planar) gold surfaces because of exactly the convolution effects they point towards elsewhere in their paper. But these are relatively minor points. The imaging and associated analysis are carried out to a very high standard, and their (sub)molecular resolution images are compelling.

As Zhou et al. point out in their paper, STM (or atomic force microscopy) of nanoparticles, as compared to imaging a single crystal metal, semiconductor, or insulator surface, is not at all easy due to the challenging non-planar topography. A number of years back we worked with Marie-Paule Pileni’s group on dynamic force microscopy imaging (and force-distance analysis) of dodecanethiol-passivated Au nanoparticles. We found somewhat similar image instabilities as those observed by Zhou et al…

A-C above are STM data

A-C above are STM data, while D-F are constant height atomic force microscope images [3], of thiol-passivated nanoparticles (synthesised by Nicolas Goubet of Pileni’s group) and acquired at 78 K. (Zhou et al. similarly acquired data at 77K but they also went down to liquid helium temperatures). Note that while we could acquire sub-nanoparticle resolution in D-F (which is a sequence of images where the tip height is systematically lowered), the images lacked the impressive reproducibility achieved by Zhou et al. In fact, we found that even though we were ostensibly in scanning tunnelling microscopy mode for images such as those shown in A-C (and thus, supposedly, not in direct contact with the nanoparticle), the tip was actually penetrating into the terminating molecular layer, as revealed by force-distance spectroscopy in atomic force microscopy mode.

The other exciting aspect of Zhou et al.’s paper is that they use pattern recognition to ‘cross-correlate’ experimental and simulated data. There’s increasingly an exciting overlap between computer science and scanning probe microscopy in the area of image classification/recognition and Zhou and co-workers have helped nudge nanoscience a little more in this direction. Here at Nottingham we’re particularly keen on the machine learning/AI-scanning probe interface, as discussed in a recent Computerphile video…

Given the number of posts over the years at Raphael’s blog regarding a lack of rigour in scanning probe work, I am pleased, and very grateful, to have been invited to write this post to redress the balance just a little. SPM, when applied correctly, is an exceptionally powerful technique. It’s a cornerstone of nanoscience, and the only tool we have that allows both real space imaging and controlled modification right down to the single chemical bond limit. But every tool has its limitations. And the tool shouldn’t be held responsible if it’s misapplied…

[1] Unless we’re talking about scanning near field optical microscopy (SNOM). That’s a whole new universe of experimental pain…

[2] This is the “zeroth” order approach to simulating STM images from a calculated density of states. It’s a good starting point (and for complicated systems like a thiol-terminated Ag374 particle probably also the end point due to computational resource limitations) but it is certainly a major approximation.

[3] Technically, dynamic force microscopy using a qPlus sensor. See this Sixty Symbols video for more information about this technique.

 

3 comments

  1. 1. Ever think about just doing regular chemistry/physics instead of surface science?

    2. Can you use these instruments to learn something interesting about catalysis surfaces (since heterogenous catalysis is a surface effect and often you have some stuff on top of other stuff? Just some good manly petroleum refining or automotive catalysts. Not those wimpy gold balls with organosulfides stuck to them (that don’t really do any industrial chemistry)?

    3. Could you do anything useful with these AFM/STM/SPM thingamabobs to look at shale rock core samples (fracking stuff)? The pore system in those rocks is super tight (some of them, the diameter of a methane molecule). Yes, there are SEM images, but just wondering if some alternate techniques (yours) could be useful there. Always good to look at a system with a different tool.

    Like

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