Possiamo fotografare un atomo? Tecniche di microscopia

Hi guys, I'm Marco, for the friends Il Cuso, welcome to a new episode of the physics that you do not expect. Look at this photo, and in particular that bright dot in the middle. This photo was taken by David Nardlinger of the University of Oxford and won the first prize to a scientific photography competition awarded by the English Engineering and Physical Science Research Council. That dot is a single atom of shit suspended in an ion trap. Is it correct to say that we are looking at an atom? We will get there, because in this video we will try to answer this question. Can we photograph an atom? Ah, before continuing, I have three news for you. Two festivals, a project that is no longer too secret, but we'll talk about it at the end of the video. There would be a lot to discuss about what it actually means to see an atom and consequently photograph it. From the moment we are used to considering photographs, even the spectacular images produced by the James Webb Space Telescope, which are actually false color re-elaborations of an image obtained in infrared, that is, in a part of the electromagnetic spectrum that we cannot see. Let's take it easy, let's assume that any signal coming from a device that can be converted into a map with different colors is enough to call it a photograph or at least an image of the object we are investigating. Let's start with the most elementary case. When we want to see something too small better, we can use a magnifying lens or a microscope, which work the same way. The rays of light and the reflection transmitted by two points very close to an object are separated by a convex lens, until they are far enough to allow our eye to distinguish them. With the naked eye we can generally distinguish two points on a sheet 25 cm from us, if they are at least a few tens of microns apart. From what I've said so far, we might think that just having a fairly large lens gives us a sufficient number of incident photons, because let's not forget that separating the rays always means redistributing the photons on a larger surface, and we could also be able to see an atom. But things are not that simple, otherwise this video would already be over, and instead you can see it well that the bar below is far from the end. In optics there is a well-known limit to the maximum magnification that we can obtain through lenses, and this was defined by Rayleigh, who came to establish that in a system of ideal lenses, the only limit to magnification is given by the diffraction of the iris, and here by iris we generally mean the opening through which the observation is made. There is a very famous relationship between the minimum angle under which two points can be observed, the wavelength of the light used and the width of the iris. From the moment that even the relationship between the angle under which these two points are seen, their separation and the distance from the observer is known, we can come to define the minimum dimension of an object that is, let's say, a few centimeters away from us using the smallest wavelength visible to our eye, that is about 450 nanometers or billionths of a millimeter. This means that the maximum magnification possible for an optical microscope is around 2,000 per, that is, the distance between two points is increased by 2,000 times. Our eye can distinguish at most points from which separation is at least a few tens of microns. Is it enough to hope to see an atom? Thanks bell. Unfortunately no, since the typical dimensions of an atom can be derived from the distance of these little ones in a crystalline reticle and the value we obtain is about 0.1 nanometers and this number multiplied by 2,000 times is only 0.2 microns. However, we could think of reducing the wavelength, therefore exploiting a luminous source that allows us, obviously using an appropriate sensor, to go down those three orders of magnitude that we would need to investigate the atomic structure. And this would mean using radiation with a wavelength of the order of the nanometer. It's better or less. We are thus in the field of X-rays. X-rays can be easily seen with appropriate detectors, but there are two problems, at least. The first is related to the extreme energy transported by these photons, which can easily lead to ionization and dissolution of chemical bonds. It is difficult to observe something while it is literally being done in pieces. The second problem is related to the resolution of the detector, that is, to the minimum distance that the eye that observes the X-rays is able to evaluate. Unfortunately, X-ray detectors, like sparklers, are quite coarse from this point of view. They work according to a rather simple principle. The photon interacts with a surface of the detector by extracting an electron from it. This electron is too little to be detected, so it must be multiplied. To do this, it is accelerated by a great difference in potential and pushed against a metallic surface, which for the energy of the impact generates other electrons. The process is repeated many times, each time increasing the number of electrons until it reaches a fairly intense current to be measured. The problem is that it makes no difference where the photon hit the surface. The signal at the end of the multiplication cascade is always the same, and since this surface has a spatial extension certainly higher than that of an atom, we have no way to understand precisely where those photons were reflected. You may have heard of a very recent news according to which the first X-ray of a single atom was published. But we'll get back to that later. Before completely abandoning optical microscopy, I would like to present you with a technique that could be considered as borderline. In fact, I remember that when I studied it during a master, there was a certain discussion about the fact that it could correctly be defined as an optical technique. I'm talking about the Scanning Near Field Optical Microscopy, or SNOM, sometimes called NSOM, but this acronym is literally unpronounceable. To understand how it works, you first need to explain what the so-called evanescent waves are, another name for the near field. You have an evanescent wave when an electric or magnetic oscillating field does not behave like an electromagnetic wave, in particular it is not able to transport energy. These oscillations remain located in the vicinity of their source and their amplitude decreases exponentially, therefore very fast. Hence the names of near field and evanescent wave. An effect you may have witnessed and that involves the existence of an evanescent wave is total internal reflection, that phenomenon that occurs when a ray of light that travels in an optically denser medium, that is, with a higher refraction index, such as water, hits the surface that separates it from a medium with a lower refraction index, such as air, with a sufficiently large angle, which in turn causes the total reflection of the luminous ray instead of the most notable refraction. However, in a case like this, Maxwell's equations establish conditions for the values ​​of the various fields in the various directions. In the direction parallel to the separation surface, the magnetic field must be an evanescent field, otherwise part of the energy of the incident ray would be transmitted beyond the interface, which would violate the principle of energy conservation. The case that interests us here is when a laser beam is propagated in an optical fiber that at a certain point has a friction that reduces the section below the wavelength. Electromagnetic radiation cannot propagate beyond friction, but in the immediate vicinity an evanescent field is generated, which can be reflected by a surface, if it is quite close, and recovered by the same probe that emitted it. This optical fiber can also be included in an AFM tip, which I will talk about in a little while. But if we have already said that optical microscopy is subject to the limits of Rayleigh, what advantage could we ever have in complicating in an exaggerated way what could be much simpler? The reason is that the evanescent wave must not respect the limits of Rayleigh, which instead concerns only electromagnetic waves free to propagate. In a SNOM, it is the width of the opening that determines the maximum resolution and mechanically fibers can be obtained from a very small diameter. The lateral resolution of a SNOM can therefore reach values ​​below 5 nanometers, something that would require radiation in the field of extreme ultraviolets, if not directly in the X-rays, while a SNOM can easily achieve those results with radiation much more ... ordinary. However, we are still using an optical source and we have seen that going down to the atomic resolution is not possible, even if we can get very close. So let's try to find a way to overcome this problem, abandoning photons and using a different probe, the electrons. Electrons have many advantages over photons, they are relatively simple to produce and much easier to control their kinetic energy, which is relevant in the speech we are going to do, and being charged particles they can be directed almost at will through electromagnetic fields. The principle of operation of an electron scanning microscope or SEM, from Scanning Electron Microscope, is quite simple. A well-collimated beam of electrons, either well focused, is directed against the surface of the object we are observing. The electrons produced from the surface, which we will investigate in a moment, are collected by a suitable detector and the signal is transformed into an image. From the moment the beam of electrons is really well collimated, it is necessary to move the beam along the surface to be able to have a comprehensive image. And from here it comes to the term scanning. First of all, how do we produce electrons? The simplest way is through a thermionic cannon, which is not a weapon of mazinga, but a rather simple electronic component. In fact, it is a metallic filament passed by an electric current. You know that when a wire is passed by a current, this heats up, this phenomenon is called the Joule effect, and it is at the base of electric bulbs and is a secondary effect in old incandescent bulbs. But the filament does not emit only heat. The less connected electrons can in fact free themselves from the metal's potential barrier and are launched into the surrounding environment. At this point, just put a positive charge electrode at a certain distance, that is, an anode, and the electrons will be accelerated towards it, reaching a kinetic energy that depends on the potential difference between anode and filament. In the anode there is a hole and the electrons that intercept it pass through the electrode and form a first focused band. An alternative method is field emission, called FE, from the English term Field Emission. The microscopes that use this method of emission are therefore called Phasem. Electrons are extracted only from the potential difference, which can also be relatively small, and are much more monochromatic, that is, they have a distribution of energy and kinetic energy much more narrow. At this point, through further magnetic electric fields, it is possible to further focus the band of electrons, obtaining a spot that can be very very small, even less than a nanometer. In general, it is necessary to work in conditions of ultra-high vacuum to make sure that the incident band of electrons, but also the electrons produced by the surface, are not disturbed in their motion. However, in the 80s, SEMs were developed that were able to operate at pressures much closer to the atmospheric one, or in very soft voids. They are called ESEM, from Environmental SEM, and are ideal for samples that tend to sublimate or evaporate under vacuum. When the primary electrons hit the surface, there are several possible outcomes. Some electrons will interact with the electric field of the atomic nuclei and will undergo what is called elastic scattering, that is, they are deflected from their original direction, but their energy remains substantially unchanged. This is due to the great difference in mass between the nuclei and the electrons. And elastic scattering can deviate the electrons significantly, until they go back, a phenomenon known as backscattering. Alternatively, the band's electrons can interact with the sample's electrons and undergo unelastic scattering, that is, they are deflected and lose a certain amount of their kinetic energy. In this case, the deflection is tending to be less. If our microscope observes the electrons in backscattering, we will talk about SEMs, while if we look at the electrons transmitted through the sample, we will talk about Scanning Transmission Electron Microscope, or STEM, or in short, TEM. Given the small deflection suffered by the transmitted electrons, the TEM manages to have resolutions practically equal to the width of the spot. But what are these resolutions? Typically a SEM at a resolution of the order of 5-10 nanometers, but there are versions at high resolution that can go down to less than a nanometer, so they are able to observe small molecules. A TEM can also go down an order of magnitude, so it is able to see atoms. And finally, here we are! But it's not all gold what shines, unless it's a sample prepared for the SEM. In fact, the preparation of the sample is essential to be able to correctly observe a sample in a SEM or a TEM. In a SEM, it is necessary that the sample is conductive enough to be able to discharge the electric charge given by the continuous flow of electrons to the ground, and therefore it is often covered with gold or another highly conductive metal, if the sample is not alone. In this case, we must consider this technique as destructive. In a TEM, on the other hand, the sample must be incredibly thin to avoid first of all that the electrons cannot cross it for absorption, and second , because the electrons undergo the least amount of multiple shocks while crossing the sample. We are talking about thicknesses less than 100 nanometers, definitely not simple to obtain and not suitable for all samples. Before moving on to a completely different type of microscopy, I would like to mention a method that I honestly did not know, and which is in fact the reason why I made this video. Some time ago, I don't remember exactly where, I received a comment in which I was asked to talk about Pticography. I re-read that name several times because it seemed so strange to me. However, doing a quick research, I discovered that it is a technique of microscopy used to obtain images at very high magnifications, thanks to the possibility of using not only visible light, but also X-rays and electrons, and therefore I think it can be good at this point of the video. Since a video only on this topic would have been a little bit, then I decided to get into other techniques that allow to enlarge the image of an object, hopefully getting to look at the single atoms. Pticography is a strange technique, however, compared to the ones we have seen. The main difference is that the coarse data obtained by this method does not look like an image of the object at all. Let's try to understand something. The image is obtained from a profile of interference. The photon beam, or other particles, passing through the sample, undergoes a certain profile of scattering, that is, it is diffused into space, and it can also undergo interference, as happens for example in another technique that I have not told you about, which is X-ray diffraction. The image obtained, however, is only the figure of interference produced by the point of the sample crossed by the beam of particles. If we now move the sample slightly to the side, the figure of interference produced by the point next to the first will be a little different, and in part superimposed on the first. This operation is repeated by moving the sample and collecting the images of interference generated by the various points close to each other. Well, and now? Now we need a complicated algorithm that allows us to reconstruct the original image, that is, the structure of the observed sample, starting from all these figures of interference. This is a complex operation that presents a fundamental problem. We are not looking at an image of the sample obtained by interaction with a few particles, but in fact an image reconstructed by an algorithm. In the chapter dedicated to the ptychography of the Handbook of Microscopy, which I have used as a source for a good part of this book and whose references you will find in the description, an affirmation is made that I find very funny. The data usually do not look like the object of interest at all, we must completely trust an algorithm that provides us with the result, which makes some scientists more nervous. But then, why does ptychography still exist? Three important advantages of this technique are presented. First of all, it does not require a lens to concentrate or diverge the beam of particles, which is extremely useful, especially when it comes to X-rays, so it was very expensive to build the right lenses. Now the costs have decreased, ptychography still uses X-ray lenses to improve resolution, but these are not necessary to obtain excellent images. Second, this technique is great for studying transparent objects that absorb little, since in addition to the intensity of the radiation transmitted, its phase can also be measured, which allows you to obtain the original image even if with algorithms far from banal. And finally, and I find that this is a very interesting property of this technique, it is self-calibrating, in the sense that any aberration or optical defect can be corrected through the algorithm and eliminated by the final image. It seems to me, but maybe I'm wrong, something similar to what our brain does with the blind spot of our eyes. Using the image coming from the other eye, which for perspective reasons can see what is in correspondence with the blind spot, our brain reconstructs the complete image so that we have to do a little elaborate operations to realize that each of our eyes actually has a completely blind spot. If you want to deepen the topic, there are countless articles and videos online. I point out those of Jacopo on his Instagram profile, you can find it below in the description, but now it's time to move on. How do we expand the plethora of champions we can see the atoms of? We can try to move on to techniques that do not require a beam of particles for measurement, and the two main techniques of this category are the AFM and the STM. These are technologies based on an approach that may seem very peculiar, given what we have seen so far, passing a very thin stilo over a surface. Maybe it may seem crude, but these are techniques that are widely able to see the atoms. Moreover, the AFM is a machine that I am fond of, since for my thesis of degree I used this tool. Let's start with the atomic force microscope, or AFM, from Atomic Force Microscope. The probe is a tip that is literally grown, layer by layer, on a bar called in jargon cantilever. This tip can be sharpened to the point of being considered as monoatomic. There are some on the market in which, on the top of the pyramid, a tiny column is deposited of carbon atoms, and therefore the tip is literally a single atom. Well, but what do we do about it? When the tip is brought close to the surface, an interaction is established between the reciprocal atoms, and we can take advantage of this interaction in three ways, which are always called in jargon tapping mode, not contact mode, and contact mode. The third is the simplest, not different from what we do when we pass a finger on a surface to feel the roughness. The tip is put in contact with the surface until it exerts a pre-fixed force. This involves a flexion of the cantilever, which is kept constant by the machine while the tip is moved along the surface. So the tip is moved up and down to follow the corrugation of the sample. A corrugation that, as I said, can be as small as an atom. Although this method can work at atmospheric pressure and even under liquid beating, there are ultra-high-void AFMs capable of scanning small surfaces so quickly that it is possible to see the rearrangement of the atoms as a result of thermal effects or mechanical stress. You may have noticed that I did not go into detail about what it means at the atomic level to keep in contact, and I don't think I will do it in this video, since it seems to me that it is already being seasoned enough, but it is something that has to do with electromagnetic forces exerted between the atoms. The non-contact mode is a little more complicated. The cantilever is oscillated at its resonance frequency, but the amplitude and the displacement with respect to the forcer do not depend only on its mass and shape, but also on the forces with which the tip interacts with the surface. The tip is brought as close as possible to observe an alteration in the amplitude, a sign that the tip and surface feel each other, and then the scan is performed. Maintaining the oscillation amplitude requires moving the tip up and down, always following the roughness of the surface. The tapping mode is not very different, but the tip comes into contact with the surface at each cycle, a bit like sticking and detaching the tip. These oscillating modes have a big advantage. The displacement between the forcer and the tip depends, among other things, on the structure and composition of the surface, and this allows to discriminate between different areas of the sample. Ah, if you are curious, my thesis is oriented to study the friction coefficient on the scale of the micron or lower. The T of STM stands for Tunneling, since the tunnel effect microscope or scan uses one of the most extraordinary phenomena foreseen by quantum mechanics. I know you've heard this story a million times, I've talked about it several times too. The tunnel effect has an interesting role in the history of quantum mechanics, since it is a very counterintuitive phenomenon that was first foreseen theoretically and only later demonstrated at the experimental level. Moreover, it is the same phenomenon that allows some atomic nuclei to undergo alpha decay, as I describe in my book Radioactivity, but I'll talk about this later. In short, the tunnel effect can be verified when, between two areas where a certain particle can be found, a third is found where the particle cannot be found. There are countless examples that can describe this situation. For example, the potential energy of some molecules, with particular symmetries such as ammonia, presents two minima in which the nitrogen atom can be found, which are separated from a local maximum. Or, which is the case that interests us, two conductive materials in which there are almost free electrons, separated by a layer of insulating material. In this situation, something really unusual can happen. Although a particle that is on one side of the wall does not have enough energy to overcome it, it is still possible, following an observation, to find it on the opposite side. This is because, despite the presence of the wall, the function that describes the state of the particle, its wave function, must maintain some properties and this means that it has a tail that extends beyond the wall. And since the probability of finding a particle in a place depends on the width of the wave function, the fact that the latter has a value different from zero means that the probability is different from zero. It is difficult to give a macroscopic equivalent of this phenomenon, because if we think of a ball confined to one side of a wall, we have no other way to get it to pass on the other side except by giving it enough energy to overcome the wall itself. But this is not what happens at the quantum level. It is a complicated thing, in short. In an STM, a tip similar to that of the FM is used, but with some important differences. First of all, the tip must be conductive, or at least covered with conductive material, and the same must be true for the surface to be investigated, so we have some limitations similar to those of the SEM we talked about earlier. Secondly, the interaction between tip and surface is given by a potential difference generated between the two conductors. Between the two, which are not in contact, there is an insulator, which can be air or vacuum. Classically speaking, despite the difference in potential, you should not observe any passage of electrons from one conductor to the other, precisely because in the middle there is the insulator. In reality, however, if the distance between tip and surface is quite small, some electrons will be able to jump through the tunnel effect from one conductor to the other, and usually the potential will be oriented in such a way as to pass the electrons from the tip to the surface. The reason why STMs are so accurate in observing height variations at the atomic level is the fact that the intensity of the tunnel current varies exponentially with the distance, and therefore small variations in distance correspond to a significant variation of the current. STMs also have different modes of use, but more obvious, constant current and constant height. In one case the tip is raised and lowered to keep the current constant, in the second case the tip is kept at a constant height and the current variation is observed. Even in this technique it is possible to notice differences in the composition of the surface, since the energy required to extract the electrons varies from material to material, since it was not necessary to cover it with gold before. Well, now you know a lot of techniques that can even get us to see single atoms, but there are a couple of questions left unanswered, and the first is the photo of Nadlinger that I showed you at the beginning of the video. What are we seeing? Is that really an atom? Let's say ... no? In the sense that that bright dot is actually visible light to the naked eye generated by a single atom. But let's see the context for a moment. The atom in question is a shit atom, or rather a positive shit ion, that is, an atom to which at least one electron has been removed. Having given a clear charge to the atom is essential to keep it still, since it is in an ionic trap, that is, a portion of space in which the properly positioned electric fields create a potential hole in which the ion in question remains blocked. Nadlinger works on the immobilization of atoms for quantum computing, so he is familiar with this kind of operations. The fact that the atom is almost immobile is important for our speech. At this point the atom has been illuminated with electromagnetic radiation in the field of blue-violet. The photons have excited the electrons of the atom, which subsequently fell back into their fundamental state, emitting a photon with the same energy. This is what we usually observe with the phenomena of fluorescence and phosphorescence, but in general we have to deal with billions of billions of billions of atoms, so it is inevitable that the photons with which we hit our fluorescent target go to a sign, that is, they find an atom to excite. Here there is only one atom. Consequently, it was necessary to take a photo at very long exposure to be able to collect a sufficient number of photons re-emitted by the atom of shit. If you want, then, with the naked eye, we will not see anything, since our eye is not sensitive enough to see a single photon, while here we have collected a lot of them, emitted in different moments. Then understand why it is so important that the atom remains as immobile as possible. If it could swing around, the image would have come to life, since once the photon would have arrived at a certain point, the next time at a slightly different point. Perhaps we cannot say that we are seeing a single atom with the naked eye, but for sure it is a spectacular image, which required quite ordinary optical equipment. Although an infrastructure behind it is definitely less ordinary. And before the final news, I wanted to tell you about the first X-ray of a single atom. In May, a group of scientists led by Professor So-Wai La of the University of Ohio published a paper in which they claim to have obtained for the first time the X-ray of a single atom. Let's say that things are slightly different from how they were told, but the news is very interesting. The X-rays, having the ability to excite even electrons very close to the nuclei, give us the opportunity to discover not only the crystalline structure or less of the material, but also recognize it in the composition, because the electronic spectrum is characteristic of each single element. However, since the signal emitted by a single atom is very weak, this puts a limit lower than the minimum number of atoms that could be studied, about 10,000. Below that number, the signal is so weak that the common X-ray detectors can't read anything. And this is where the work in question comes in. X-rays have been produced, as is now quite commonplace in this sector for several years, through a synchrotron, that is, an acceleration of circular-shaped particles. The synchrotron light, produced by the charged and deflected particles in the ring, has an important feature of being very monochromatic, that is, having a single wavelength, a property that, on the other hand, medium-radiograph tubes do not have, also because it is not necessary. These rays hit and excite a small group of atoms, and to be able to see the signal of a single atom and reconstruct its spectrum, a detector is used that I have already told you about, a tip for STM. The signal read from this tip is activated by the absorption of X-rays by the atom, so by varying the incident female length it is possible to reconstruct the spectrum of a single atom and consequently identify it. This video is about to end. Finally. But give me a couple more minutes. I know it was a long walk through microscopy, but I have three important news to give you. The first is that ... well, I also wrote a book. It will be entitled Radioactivity, it will talk about all the aspects related to this phenomenon so complex and fascinating, and it will come out, if all goes as it should, in autumn. I'll give you more details in a bit. The second is an event that, if you are from Bologna and around, you cannot not participate. 2023 is the tenth anniversary of the Foundation of the Association of Scientific Disclosure, Minerva, of which I am a proud member, but also the tenth anniversary of the well-known site Bufala e un tanto al chilo, or Butac, directed by my friend Michelangelo Coltelli. But instead of making you a gift to them, they decided to make them a gift to the citizenship of Bologna. On November 10 and 11, in Sala Borsa, one of the most beautiful libraries in Bologna, a two-day meeting will be held entitled This Order, 10 years of correct scientific information under the two towers. There will be a lot of guests, but I will tell you more about this later. You can, if you want, support the event with a small donation. I'll leave you the link below. Finally, always in November, but the following weekend, on the 18th and 19th, there will be the Milano Eco&Brain, the Festival of Rational Environmentalism, organized by the pages referring to the Italian project for nuclear energy, including the physics that you do not expect. This will also be an opportunity to talk to numerous guests about numerous aspects related to the environment, energy, sustainability, agriculture and farming of the future, and much more. Well guys, this video ends here. If you liked it, as usual, thumbs up and share it with your friends. Thank you for watching, I'll see you in the next video, which will be a little different from the usual and I recommend, be curious. Physics is where you don't expect it.