I’ll tell you about electrochemical metamaterials which is actually absolutely new term, not yet accustomed, not yet established. Before understanding the idea of electrochemical metamaterial, you should realize what metamaterial is. Metamaterial is material with some odd funky disruptive properties, properties that people didn’t have before, that don’t arrive naturally as it is. The whole duration of metamaterials first came out with photonic metamaterials: it means there are some materials which will have very very unusual optical properties, for instance. This Univ is one of the leading groups in the world in terms of the most elderly metamaterials love invisible cloaking: that’s a grouping of John Pendry and some people who are working with him on how to create the material a perfect lens, so the light will be passing by, but it’ll not look you, you’ll disappear the vision. It’s a still a long way in realization of this kind of mathematical dream which was predicted first theoretically.
That’s just one example. In principle, if we’re talking about metamaterials that may be not only optical metamaterials, may be electromechanical metamaterials, related with, for instance, robotics, they usually aren’t called metamaterials, they’re called bright materials: the materials that can do some comic functions at your own will.
Why electrochemical? Electrochemistry is essentially a science, a very elderly science, which can be reduced to the following thing: you’ve an electrode and electrolyte, and when you polarize that electrode, you charge it, then there will be counter ions on the opposite side, the charged electrode will arrive there and build what’s called electrical double layer or space charge in front of it. And then your electric field which is due to polarization of that electrode will be localized in very narrow space of the thickness of that double layer. Generally this depends on concentration of electrolytes and it can be one nanometer or more. But then what’s happening, within one volt you can have a field which will be love potential of one volt per one nanometer. This is very challenging to in physical systems: only recently, when people started to work with nano gaps, that were able to attain such fields.
So what’s exciting about having such fields? In electrochemistry there was nothing exciting, it was everyday reality for them beca they were using that potential, first of all, to drive electrochemical reactions. You polarize the electrodes, for instance, positively, and negative ions are coming to the surface, and then they’ll get portion at the electronic change, it’ll be what’s called Faraday process or electrochemical reaction. That was one kind of application. Other kind of applications are batteries or super capacitors and many other things. So that was standard there, but to those abilities to dramatically modify the properties of a material, whatever it is, that’s a new direction, new turn how electrochemistry can be d. We’re accustomed that it’s batteries, it’s fuel cells, and so these are standard things or reactions or polishing of metals and all those applications. But it may have actually other applications as well.
Presently let me speak about photonic metamaterials, or even more narrow, the so-called plasmonic metamaterials beca that’s how actually photonic metamaterials started. What’s the science of plasmonics? When it’s all started? Long time ago. It was approximately in one thousand nine hundred-seventieth when the people love Andreas Otto in Dusseldorf and Kretchmer suggested certain geometries of prisms that they can focus light in such a way that by shining light on the metal surface then they can excite the so-called surface plasmon.
So first of all, what’s it plasmon? Plasmon is a collective excitation of electrons. It’s nothing very complicated for an expert to understand. What’s metal? You’ve a skeleton of positive ions and you’ve more or less free electrons, a sea of free electrons floating there, and this is why they conduct. When you apply voltage at the two sides of your piece of that metal, you’ll thrust those electrons going opposite to the direction of the of the field.
But plasmons are excitations when locally electrons will obtain kind of concentrated and then in another space depleted, so that waves love an acoustic wave will propagate in that system. Of course, as every wave, it’s its own law of propagation, its frequencies related to its wave vector or momentum, and that’s called dispersion relation. That dispersion relation for plasmon is such that you cannot directly excite it by shining light on it beca you cannot satisfy both conservation of energy and momentum. Those two people (maybe there were more but these are the most brilliant examples) d these setups they can crack that kind of a relationship that you cannot excite them. They were able to excite plasmons and study those plasmons, so-called phenomenon of attenuated total reflection: the light is shined and it’s not reflected in full beca it excites a plasmon, and so you can look then that plasmon.
And that was more or less academic investigation. I may be incorrect beca I’m not very much bothered about what was at that time, but approximately, if I’m not mistaken, one thousand nine hundred seventy-seven there was a discovery of giant Raman effect which was called surface-enhanced Raman effect. What’s it that Raman effect? First of all, what’s Raman effect itself? Raman scattering is a nonlinear process when you shine light with certain frequency and you then will disperse light with a different frequency, and the disagreement between those frequencies will be due to interaction of that light with some vibrational modes, and precisely the disagreement in the frequencies associated with a quanta of those modes is how you can obtain excited. That’s a feeble process. It’s a nonlinear process, so probability of this process is very low. When Raman scattering was discovered it was discovered long time ago by Raman and Krishnan in India and by Mandelstam and Landsberg in the Soviet . But Raman and Krishnan did it in solutions, their probes on which this light is kind of scattered, they were in solutions, and the Mandelstam and Landsberg did it on crystals, there was vibration of the lattice, and you can look it there.
There is a large disagreement in terms of applications: who really cares about those crystals. But in the case of Raman that’s a whole world of chemistry. Why? Beca water isn’t transparent in infrared region, an infrared region correspond to vibrations and molecules, and all chemistry is about those vibrations of molecules. So if you cannot go and directly probe those vibrations, you lose quite a lot, but using Raman scattering you can do it, you can go through transparent window, then reflect through transparent window, and the disagreement in frequency will probe this kind of challenging range of frequencies which are associated with chemistry. The legend was very dreary for the Soviet side at that time beca Raman got the Nobel Prize (actually got it alone without Krishnan). I don’t know whether it’s true but it’s a legend that he bought his ticket to Stockholm a year before the Nobel price was announced beca it was cheaper to a half if you purchase it earlier, you’d to get a ship at that time. He really wanted it very much. For that reason the Soviet people don’t (or didn’t at least) the duration ‘Raman scattering’, they ‘combination light scattering’ which was introduced by Landsberg and Mandelstam but it didn’t go any further love that one.
But in order to indicate this effect Raman had to very concentrated solution of those analytes he wanted to study love that beca the signal, what people call the cross-section of the scattering, is very small, so they’d to have a lot of that stuff there. Is it very interesting? For some applications it may be fascinating enough. But what’s very fascinating to detect the fingerprints (the spectral lines of those molecules are always unique: they’re their fingerprints, people call it Raman fingerprints of a molecule), so if you can detect very tiny no of such molecules, in that case you’ll be able to detect the presence of those molecules we don’t wish to be here, including gases, poisons, any illegal substances, things love that.
Direct Raman effect as it was didn’t give that option, so what was discovered, the so-called surface enhanced Raman effect, it means people saw a coarse surface of metals and there they’ve seen very large enhancement of that signal. It was first observed in Southampton by Martin Fleischmann and two of his colleagues, Hendra and McQuillan, and they misinterpreted that result: they thought it’s beca there is a lot of stuff on this coarse surface. But later on Van Duyne in the United States has proved that if you measure that surface and compute how much stuff you can absorb there, it’ll not work beca enhancement was sometimes between 108-1012 orders of magnitude. And then he and many other scientists came up with an idea that this is an excitation of surface plasmons, and then in these gaps and cracks there will be hotspots for electromagnetic radiation beca Raman scattering goes as a force power of electric field, electromagnetic radiation, and so if you enhance it for three orders of magnitude, you’ll get one hundred nine orders of size of Raman signal. That was in seventieth and eighty. I was at one conference with people discussing it till late midnight what was the nature of that.
Then came a new era of nanotechnology. Nanotechnology of ninety started with that you can construct a lot of structures with nano resolution, different architectures, and if you can construct them, you must’ve an idea what for you’re building them and which kind of effect that’ll be giving you. One of them was to construct to work, for instance, with nanoparticles. So if you’ve these nanoparticles in some proximity to each other, you fall light on it, then beca there is a localized plasmon resonance, there is a resonance phenomenon, so you’ve a very high electric field in between. That’s called the hot spots. And then those hot spots, how we can them? First of all, you can them for surface enhanced Raman effect: if analyte molecule will obtain in that whole spot so you can then detect enhanced Raman signal of that.
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