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Surface dynamics of charge carriers measured using an indirect technique

This is a quick follow-up on a previous post related to atomic manipulations with a scanning tunneling microscope. A few days ago Kris published a part of her work in Nature Communications — "Initiating and imaging the coherent surface dynamics of charge carriers in real space", wohooh, congratulations! I paid a visit to their lab back in 2013 (the time when she had a microscope with a gigantic chamber full of "dust", sadly no picture here) and in 2015 (when her group moved into the optical spectroscopy lab with a new high-performance mini-chambered microscope). I thought I'd continue the tradition of writing blue-sky nonsense on their work and put a few words from an ordinary person's perspective.

Apparently, plugging-in and out atoms with the tip of an STM is a lot more complicated than it looks at first sight. The tip itself is a source of charge carriers, as that's how the STM works. When the tip hovers above the scanned sample it causes carriers to tunnel through to the surface of the sample. Naturally, such charges can lead to a counter-reaction and disturbance of the spatial positions not only of the atoms directly underneath the microscope's tip, but also in a quite large radius around the tip itself. This paper describes the initial (at the start of injection) charge carrier dynamics on the scanning surface, which have been deduced by looking at the scattering patterns of toulene molecules. Oh well, or at least that is what I understood :)

The first figure in the paper shows images of Toulene molecules scattered on the surface of a Si(111) sample. A comparison between images before and after (direct?) atomic manipulations is shown. If one takes a bird's eye view on the Toulene molecules before and after the manipulation attempt, it seems like after the tip's bias has settled and reached a steady state, the Toulene molecules move in a systematic direction, that forces them to lump to each other. I.e. my interpretation is that in the before images Toulene is a lot more scattered, as compared to its position after manipulations. This change of position of the Toulene molecules on the surface of the sample is a source of information about the charge carriers' path (?) and hence behaviour. Right after the charge carriers reach the surface of the sample, they encounter a number of scattering events, before reaching a steady state (steady state = very vague expression). During that transitional time the carriers transfer their energy to the atoms on the surface, which leads to their movement and bond breaking. This movement gives indirect information about the dynamics of electrons/holes themselves.

Surely this paper is a lot more involved and is waay beyond my level of understanding of quantum physics to be able to summarize it in my own words which make sense. But, what really fascinates me — and that's why I started writing this ambitious post — is mankind's achievements in material science to this date. I mean, these things are roughly ~1 nanometer scale, and the hole/electron dynamics described occur in the order of femto seconds... i.e. that's easily deep inside the THz range. And when you add ambient temperature effects the picture becomes a complete mess, yet, it is quite well understood. On the other hand, carrier dynamics in the order of ~100s femtoseconds is not that fast, maybe that's why modern-day electronics is still struggling to conquer the THz gap?

Date:Sun Oct 2 14:58:51 CET 2016


12 Oct 2016, 11:55
Update: here's more on the topic with a much better explanation -- tml
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