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On August 30 2011 11:37 Ympulse wrote:Show nested quote +On August 30 2011 03:18 0mar wrote:On August 30 2011 03:15 ilj.psa wrote:
not really understand the images, if its true its amazing though electrons don't follow an orbit, ithey are pretty much at random places inside an orbital That's been known for about 100 years now... They knew about atoms in 1911? Source please.
John Dalton postulated the existence of atoms in 1805 to explain the Law of Multiple Proportions.
In 1865, Josef Loschmidt measured the sizes of the molecules that make up air.
In 1897, J. J. Thompson discovered the electron.
In 1913, Niels Bohr proposed the Bohr model, which consists of electrons orbiting an atom's nucleus, at fixed distance orbits.
In 1926, Schrödinger proposed that electrons behave as waves, instead of particles, orbiting an atom's nucleus in an orbital cloud, rather then at fixed distance orbits.
So yeah, we've known about atoms for a while. These pictures are important, because they confirm our theories.
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On August 30 2011 11:37 Ympulse wrote:Show nested quote +On August 30 2011 03:18 0mar wrote:On August 30 2011 03:15 ilj.psa wrote:
not really understand the images, if its true its amazing though electrons don't follow an orbit, ithey are pretty much at random places inside an orbital That's been known for about 100 years now... They knew about atoms in 1911? Source please.
Not exactly this, but for example the Rutherford-Bohr Atom modell is from 1913. The modern Orbitalmodel with electrons as probability areas is from 1926. The general concept of atoms is known since the 19th century. For sources, just go to wikipedia and look at their sourcelist, but you can find this in about every book about atomic models.
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I was blown away that I can look at that picture. The blurry picture. And count the nodes in the HOMO and LUMO. And see how well it agrees with theory. FINALLY! MO THEORY IS THE ONE THAT ISN"T A BLOODY LIE!
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Crazy when you're told as a kid that we can't actually see it yet, and then one day we can see it. Just makes it that much cooler.
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This is really good for Nanotechnology. I didn't think it was possible to be able to see electron orbitals clearly. Whether people know it or not, the era of Nanotech is almost upon us.
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On August 30 2011 08:13 jgoonld wrote:Show nested quote +On August 30 2011 06:49 Nycaloth wrote:
The pictures we see are taken by very advanced microcopes, the techniques used are scanning tunneling mocroscopy (STM) and atomic force microscopy (AFM). though called that, they are not microscopes in the "traditional" sense and do not use light and lenses to peek into the very small. instead, both work by scanning a surface with a very narrow metal tip and recording certain surface properties at different points. these measurements can be combined into pictures of the surface, single measurements being the pixels combining the picture. Through the use of piezo crystals and sofisticated dampening of the tip-sample system as a whole, the motion of the tip can be controlled very precisely, on scales smaller the the diameter of an atom. understanding of both techniques requires a basic familiarity with quantum mechanics, but the wikipedia articles on the subject should be enough to explain how exactly they work.
Since we are doing quantum mechanics (QM), one has to be exceedingly careful when using terms such as "position", "velocity" or "path", since they cannot usually be defined in a meaningful way any more. i cant read the full article referenced in the OP at the moment since im not at the university and dont have access to it, but what we see in the pictures are molecular orbitals. what QM can only predict possibilities and other statistical quantities. orbitals are such a quantity, they represent the possibility to find an electron within a certain volume of space. this quantity can be computed numerically for complex systems such as molecules and these days can be measured by STM. due to electronic properties, namely the pauli exclusion principle, only two electrons can fit into one of these orbitals at a time. they are filled up until all the electrons in the molecule are used up. Thus, wer get the highest occupied molecular orbital, the HOMO, and the lowest unoccupied moelcular orbital, or LUMO. these two are usually involved when the molecule exchanges charge with its surroundings, eg during the formation of chemical bonds, which makes them interesting for study.
Thanks for the insight! I don't have much knowledge of this field, but I have some questions about what they're looking at. How are pure "photos" of the HOMO and LUMO (or at least electron probability densities of electrons at these certain energies) obtained? I assume that they excite electrons into the HOMO to get the images of it, but won't there continue to be electrons in lower energy MO's? And wouldn't the same be true for LUMOs? How are they isolating electrons in certain orbitals for viewing while not sensing all of the other electrons in the molecule?
To understand how these pictures are made, one has to understand how the machine that takes them works. The scanning tunneling microscope (STM) uses the so called tunneling effect in quantum mechanics. Remember the concept of uncertainty? In QM, we can no longer say where exactly something is with certainty, instead, objects like electrons are "smeared out" over an area of space. These areas will extend beyond the conceptual limits of any given body, say a metal plate or a tip. If we approach plate and tip to one another until these areas of "smeared out" electrons overlap, the electrons can pass from one contact to the other, even though there is no electrical contact between the two. This is quantum tunneling.
By applying an electrical tension between the two contacts, we can give a preferred direction to the tunneling of electrons and measure the resulting tunneling current. This is what an STM does: by scanning a surface and measuring the tunneling current at many points, we can construct a picture of that surface much like a computer screen constructs an image from many pixels.
How does this allow us to image molecular orbitals? if we put molecules on top of the surface that is scanned in the microscope, the picture changes a bit. By varying the electric tension we apply, we can vary the energy of the electrons participating in the tunneling process. If there are MOs in the window of energy we are looking at, the tunneling current will be enhanced because the electrons dont have to pass from tip to sample in one go, but can instead tunnel in two smaller steps, into the molecule first and then from there into the surface. So by scanning the voltage as well, we will observe sharp increases in the tunneling current every time that we pass the energy of a molecular orbital! In the first derivative of the current, these steps are transformed into peaks: they show the position of the MOs in the energetical spectrum.
This is what you see in the pictures: a spatial map of the first derivative of the tunneling current measured between a metal tip and a metal surface on which molecules have been deposited, taken at the energy of the HOMO and LUMO, respectively.
hope that helps!
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On August 30 2011 19:11 Nycaloth wrote:Show nested quote +On August 30 2011 08:13 jgoonld wrote:On August 30 2011 06:49 Nycaloth wrote:
The pictures we see are taken by very advanced microcopes, the techniques used are scanning tunneling mocroscopy (STM) and atomic force microscopy (AFM). though called that, they are not microscopes in the "traditional" sense and do not use light and lenses to peek into the very small. instead, both work by scanning a surface with a very narrow metal tip and recording certain surface properties at different points. these measurements can be combined into pictures of the surface, single measurements being the pixels combining the picture. Through the use of piezo crystals and sofisticated dampening of the tip-sample system as a whole, the motion of the tip can be controlled very precisely, on scales smaller the the diameter of an atom. understanding of both techniques requires a basic familiarity with quantum mechanics, but the wikipedia articles on the subject should be enough to explain how exactly they work.
Since we are doing quantum mechanics (QM), one has to be exceedingly careful when using terms such as "position", "velocity" or "path", since they cannot usually be defined in a meaningful way any more. i cant read the full article referenced in the OP at the moment since im not at the university and dont have access to it, but what we see in the pictures are molecular orbitals. what QM can only predict possibilities and other statistical quantities. orbitals are such a quantity, they represent the possibility to find an electron within a certain volume of space. this quantity can be computed numerically for complex systems such as molecules and these days can be measured by STM. due to electronic properties, namely the pauli exclusion principle, only two electrons can fit into one of these orbitals at a time. they are filled up until all the electrons in the molecule are used up. Thus, wer get the highest occupied molecular orbital, the HOMO, and the lowest unoccupied moelcular orbital, or LUMO. these two are usually involved when the molecule exchanges charge with its surroundings, eg during the formation of chemical bonds, which makes them interesting for study.
Thanks for the insight! I don't have much knowledge of this field, but I have some questions about what they're looking at. How are pure "photos" of the HOMO and LUMO (or at least electron probability densities of electrons at these certain energies) obtained? I assume that they excite electrons into the HOMO to get the images of it, but won't there continue to be electrons in lower energy MO's? And wouldn't the same be true for LUMOs? How are they isolating electrons in certain orbitals for viewing while not sensing all of the other electrons in the molecule? To understand how these pictures are made, one has to understand how the machine that takes them works. The scanning tunneling microscope (STM) uses the so called tunneling effect in quantum mechanics. Remember the concept of uncertainty? In QM, we can no longer say where exactly something is with certainty, instead, objects like electrons are "smeared out" over an area of space. These areas will extend beyond the conceptual limits of any given body, say a metal plate or a tip. If we approach plate and tip to one another until these areas of "smeared out" electrons overlap, the electrons can pass from one contact to the other, even though there is no electrical contact between the two. This is quantum tunneling. By applying an electrical tension between the two contacts, we can give a preferred direction to the tunneling of electrons and measure the resulting tunneling current. This is what an STM does: by scanning a surface and measuring the tunneling current at many points, we can construct a picture of that surface much like a computer screen constructs an image from many pixels. How does this allow us to image molecular orbitals? if we put molecules on top of the surface that is scanned in the microscope, the picture changes a bit. By varying the electric tension we apply, we can vary the energy of the electrons participating in the tunneling process. If there are MOs in the window of energy we are looking at, the tunneling current will be enhanced because the electrons dont have to pass from tip to sample in one go, but can instead tunnel in two smaller steps, into the molecule first and then from there into the surface. So by scanning the voltage as well, we will observe sharp increases in the tunneling current every time that we pass the energy of a molecular orbital! In the first derivative of the current, these steps are transformed into peaks: they show the position of the MOs in the energetical spectrum. This is what you see in the pictures: a spatial map of the first derivative of the tunneling current measured between a metal tip and a metal surface on which molecules have been deposited, taken at the energy of the HOMO and LUMO, respectively. hope that helps!
Thanks for the insight 
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Feel pretty geeky knowing all that stuff (Chemical Engineering). Just seeing the AFM tip taken out and fitted in, and seeing what kind of images it can get of my sample.
Quantum mechanics powa!
To understand how these pictures are made, one has to understand how the machine that takes them works. The scanning tunneling microscope (STM) uses the so called tunneling effect in quantum mechanics. Remember the concept of uncertainty? In QM, we can no longer say where exactly something is with certainty, instead, objects like electrons are "smeared out" over an area of space. These areas will extend beyond the conceptual limits of any given body, say a metal plate or a tip. If we approach plate and tip to one another until these areas of "smeared out" electrons overlap, the electrons can pass from one contact to the other, even though there is no electrical contact between the two. This is quantum tunneling.
By applying an electrical tension between the two contacts, we can give a preferred direction to the tunneling of electrons and measure the resulting tunneling current. This is what an STM does: by scanning a surface and measuring the tunneling current at many points, we can construct a picture of that surface much like a computer screen constructs an image from many pixels.
How does this allow us to image molecular orbitals? if we put molecules on top of the surface that is scanned in the microscope, the picture changes a bit. By varying the electric tension we apply, we can vary the energy of the electrons participating in the tunneling process. If there are MOs in the window of energy we are looking at, the tunneling current will be enhanced because the electrons dont have to pass from tip to sample in one go, but can instead tunnel in two smaller steps, into the molecule first and then from there into the surface. So by scanning the voltage as well, we will observe sharp increases in the tunneling current every time that we pass the energy of a molecular orbital! In the first derivative of the current, these steps are transformed into peaks: they show the position of the MOs in the energetical spectrum.
This is what you see in the pictures: a spatial map of the first derivative of the tunneling current measured between a metal tip and a metal surface on which molecules have been deposited, taken at the energy of the HOMO and LUMO, respectively.
hope that helps!
You have a gift for explaining things haha. I know STM and STILL couldn't convey what it does for anybody below college chemistry! Good job
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On August 30 2011 12:21 DtorR wrote:
This is really good for Nanotechnology. I didn't think it was possible to be able to see electron orbitals clearly. Whether people know it or not, the era of Nanotech is almost upon us.
Nanotech has been with us for many years. The linewidth used to write transistors for processors is sub 100nm these days. Bit sizes on modern harddrives is sub 100nm as well. These pictures are interesting on their own, but I'm skeptical on the revolutionizing aspect.
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On August 30 2011 20:11 OrchidThief wrote:Show nested quote +On August 30 2011 12:21 DtorR wrote:
This is really good for Nanotechnology. I didn't think it was possible to be able to see electron orbitals clearly. Whether people know it or not, the era of Nanotech is almost upon us. Nanotech has been with us for many years. The linewidth used to write transistors for processors is sub 100nm these days. Bit sizes on modern harddrives is sub 100nm as well. These pictures are interesting on their own, but I'm skeptical on the revolutionizing aspect.
Single molecules are still a good deal smaller then that, on the scale of a few nm, one to two orders of magnitude lower then the bit sizes you mentioned. But miniaturisation is only one aspect of this research, maybe the most obvious. if we can build resistors, wires and capacitors from such small building blocks, electronics would get a good deal smaller yet again.
But the real prospect is efficient use of resources. Over the past few years, you may have read news articles about material shortage in the high tech industry. construction of displays, memories and chips often requires rather exotic raw materials which are rare and hard to get to and the prices have been rising steadily for a long time. If we can find new ways to do the same thing with easier to obtain materials, thats a good thing. most of the molecules looked at in current research are composed of mostly carbon, oxygen, hydrogen and nitrogen with metal centers. Thats one atom of metal per molecule.
The chemists have a unit to measure the quantity of a substance: the mole. A mole is roughly six times ten to the twentythird power, or a huge bloody lot. This is the scale on which molecules can be produced with ease with very little effort. This is the second big perspective: if we understand the dynamics of single molecules to a degree where we can build things from them, our production capabilities would be infinite for all practical purposes, while using less raw materials, less energy and probably less time then they do now.
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Nycaloth, I just want to thank you for your explanations. I only did Physics in high school but I feel smarter just by reading your explanations. This is pretty cool.
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Homo..........wat.)
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On August 30 2011 20:50 Nycaloth wrote:Show nested quote +On August 30 2011 20:11 OrchidThief wrote:On August 30 2011 12:21 DtorR wrote:
This is really good for Nanotechnology. I didn't think it was possible to be able to see electron orbitals clearly. Whether people know it or not, the era of Nanotech is almost upon us. Nanotech has been with us for many years. The linewidth used to write transistors for processors is sub 100nm these days. Bit sizes on modern harddrives is sub 100nm as well. These pictures are interesting on their own, but I'm skeptical on the revolutionizing aspect. Single molecules are still a good deal smaller then that, on the scale of a few nm, one to two orders of magnitude lower then the bit sizes you mentioned. But miniaturisation is only one aspect of this research, maybe the most obvious. if we can build resistors, wires and capacitors from such small building blocks, electronics would get a good deal smaller yet again. But the real prospect is efficient use of resources. Over the past few years, you may have read news articles about material shortage in the high tech industry. construction of displays, memories and chips often requires rather exotic raw materials which are rare and hard to get to and the prices have been rising steadily for a long time. If we can find new ways to do the same thing with easier to obtain materials, thats a good thing. most of the molecules looked at in current research are composed of mostly carbon, oxygen, hydrogen and nitrogen with metal centers. Thats one atom of metal per molecule. The chemists have a unit to measure the quantity of a substance: the mole. A mole is roughly six times ten to the twentythird power, or a huge bloody lot. This is the scale on which molecules can be produced with ease with very little effort. This is the second big perspective: if we understand the dynamics of single molecules to a degree where we can build things from them, our production capabilities would be infinite for all practical purposes, while using less raw materials, less energy and probably less time then they do now.
Yes. I realize single molecules are several order of magnitude smaller than the examples I used, my point was, nanotech has been alive and kicking for several decades. Spintronics and single electron transistors have large potentials, but however amazing these images are, they're not just going to completely revolutionize -- anything, because all engineering today is a complex multidisciplinary process where verification on the atomic level is a minor part.
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EPT
