Platinum Images

[Lynn Hepler]

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Here we develop a non-rigid (NR) registration scheme that enables us to achieve extremely high SNR images. We demonstrate sub-picometre (pm) precision measurements of atom positions in aberration-corrected Z-contrast STEM images, more than five times better than previous methods13,14, based on NR registration and averaging of an image series. NR registered images of a Pt nanocatalyst show pm-scale contraction of atoms at a ()/() corner towards the particle centre and expansion of a () facet. Combined with standardless atom counting16 to determine three-dimensional (3D) structure, sub-pm precision STEM can give new insight into the active sites controlling catalytic reactions, and the displacements from defects, interfaces or various ferroic phenomena.

where λ is a non-negative, constant parameter that controls the smoothness of the deformation, Ω is the image domain, Ω is the Jacobian of phi and 1 is the identity matrix. To avoid being trapped by local minima in E[phiNR] for nearly periodic STEM images, we use a coarse-to-fine multilevel approach. It initially registers down-sampled variants of the images, and then uses the resulting phiNR as an initial guess at the next finer sampling levels, iterating up to the original sampling. At each level, we solve the minimization problem by relating it to a time-dependent partial differential equation and using a regularized gradient descent18 with explicit time discretization and step size control19.

There is intensity consistent with one atom at the very tip of the NP corner between atoms I and K, but it does not form a peak distinct enough for fitting. We speculate that there was an atom at the corner, but it experienced large motions near its lattice site during acquisition. It cannot have been displaced from the particle by the electron beam during the series, or the intensity would be less than one atom. In the future, modelling atom vibrations at surfaces under the influence of the electron beam will be required to extract the maximum possible information from low-noise, low-distortion STEM images.

Observations of average bond contraction at flat metal surfaces22,23 can be explained by the lower coordination of surface atoms24 or the electrostatic force from surface rearrangements of the electronic charge25,26. Both explanations predict a larger bond contraction of corner atoms near facet edges and steps, consistent with our observation of bond length contraction near the NP tip. However, the varying magnitude and direction of the contraction, and the observed expansion along the () surface facet, emphasize the need for detailed microscopic investigations of real, 3D structures, not just idealized 2D surfaces.

In conclusion, sub-pm precision measurement of the positions of atoms has been achieved in STEM by NR registration and averaging of a series of short-exposure images. Picometre precision makes it possible to measure the small but crucial atomic displacements typical for surfaces, interfaces, defects and ferroic phenomena. Using this method, we have measured pm-scale bond length variations at the surface of a Pt nanocatalyst, showing large bond length contraction at a ()/() corner but significant expansion along a () surface. NR registration also enables standardless atom counting without limitation by Poisson noise, which elucidates the 3D structure of the nanocatalyst with

Owing to sample drift during image series acquisition, only a sub-area of the whole averaged image uses all the images in the series for the average. Only that area was used for further analysis. To achieve sub-pixel location of the atomic column positions, we fit a small region of pixels around each atomic column to a 2D Gaussian function plus a constant,

We at UW-Madison acknowledge the funding from the Department of Energy, Basic Energy Sciences (DE-FG02-08ER46547) in support of microscopy experiments, simulations and analysis. UW-Madison facilities and instrumentation are supported by the UW Materials Research Science and Engineering Center (DMR-1121288). We at USC acknowledge funding from the NSF grant DMS-1222390, Special Priority Program SPP 1324 funded by DFG, the Interdisciplinary Mathematics Institute and the College of Arts and Sciences in support of development and application of the non-rigid registration algorithm. We at RWTH Aachen were funded in part by the Excellence Initiative of the German Federal and State Governments.

P.M.V. conceived the study of STEM precision and nanoparticle surface relaxation. A.B.Y. and P.M.V. performed the experiments, simulations and analyses. S.I.S. and S.A.B. synthesized the Pt nanocatalyst samples. B.B., W.D. and P.B. developed and applied the NR registration algorithm. A.L. and I.S. created the relaxed Si dislocation model. A.B.Y. and B.B. drafted the manuscript, and all authors discussed and revised it.

We've already discussed Sebastio Salgado's Genesis exhibition, but this month also sees the release of 50 selected platinum prints also on display in London, at Phillips de Pury. As someone who prints with inkjet, silver and more recently alternate processes, including a recent start in platinum, I thought it would be an interesting exercise to view these exhibitions together on the same day. I have tried to approach it open minded, but realised the artisan in me couldn't help but want the platinum images to be 'better'.

So I joined the queue at the National History Museum before opening and went straight to the Genesis exhibition (another review here). This is a vast, epic exhibit, a real 'blockbuster' as I heard someone next to me remark. The images themselves focus on the natural world, encouraging us to reflect on our own lifestyles and our impact. Many images also feature indigenous peoples in their traditional dress. Salgado's inkjets are large and printed hard, very black and white yet managing to retain just enough highlight and shadow detail to make the image sing. This was no display of subtlety in the mid values, this was high impact printing. Most images work perfectly, but a few I found too 'overdone' to my taste, with a hint of an HDR type look. The inkjets themselves, are printed on a gloss paper so that the blacks are incredibly dense.

It is hard to grumble at any aspects of the exhibit, which is incredible. However, for my taste, some of the images were over large. Not only did this introduce a lack of detail and artefacts into the image, but it also means you have to stand back. In a busy exhibition, people are often walking in front of the image you are viewing, or standing close to the print, forcing you to do the same, and this did detract from the viewing experience. Salgado has tried to retain his tri-x look in a digital age, using digital contrast adjustment and the DXO plugin to add synthetic grain. It's an amazing admission to make, I'm sure many other photographers would have kept quiet about it, but the effect is incredibly true. I expected to be able to tell film from digital easily, and you can, but in a positive way. The film shots look like Kodak Tri-x on 35mm, with large areas of characteristic grain, and the digital shots have a more detailed, finer 'medium format' look about them. I went in with the mind of a sceptic, but left a convert. I've never seen digital images look so good. I left the exhibition with the words of one visitor ringing in my ears 'my faith in black and white photography is restored'.

As I was crossing London, I was a little worried. How was platinum going to compare with these amazingly impactful images? Platinum is known for its subtle shadow and highlight detail, for the richness and velvet texture of the art papers it's printed on. This would seem to be an almost antithesis of the exhibition I had just viewed.

31 Studio was charged with the task of producing the 16 editions of 50 images that are currently on display at Phillips. Platinum had a rebirth in the 1960s in the hands of Irving Penn after it had fallen out of favour after the First World War with the cost and availability of materials. It is of course known for its archival properties, as well as it's aesthetic, which is of delicacy with almost endless shadow and highlight tones. Historically the images are contact printed from negatives onto hand sensitised art paper and exposed like most alternate processes to UV light. Technological advances in digital printing allow printing negative enlargements of any size from a digital file (from a film or digital original). This new negative is used to contact print Salgado's images onto 24x16 inch Arches Aquarelle paper. This produces a matt image, which always gives the impression of a less deep black. The best platinum printers struggle to achieve a Dmax of 1.4 on platinum, whilst most of us at home with inkjets can easily achieve a deep dark 1.8 density. This would mean that the platinum prints would be remarkably different to the images at the National History Museum.

This was immediately obvious on entering the quiet and spacious Phillips gallery. With the high impact images of the Genesis exhibition at the front of my mind, I was greeted by wonderfully soft and subtle prints of a select 50 images. As I made my way around the images, I felt something wasn't quite right. Not all the images 'worked'. The high drama was missing from some of the prints. A lot of Salgado's images rely on the impact of high density prints, and whilst credit must go to 31 Studio for producing outstanding platinum work, I started to feel that maybe platinum wasn't the best media for the subject matter. What was interesting was that a few select images, that seemed over done before, now had an ability to show their true potential, to breathe a little.

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