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Author Topic: When we see an atom, what are we looking at?  (Read 9307 times)

Offline chris

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When we see an atom, what are we looking at?
« on: 13/04/2008 16:14:49 »
When IBM published their famous iconic image in 1990 showing IBM spelled out in atoms of xenon, the atoms resembled small grey footballs.

But what are we actually looking at when seeing those footballs, and how was that image produced?


 

Offline syhprum

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When we see an atom, what are we looking at?
« Reply #1 on: 13/04/2008 16:33:07 »
I believe although there is a more subtle quantum mechanical explanation we are looking at the clouds of electrons that surround the nucleus
 

lyner

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When we see an atom, what are we looking at?
« Reply #2 on: 13/04/2008 20:04:43 »
And, of course, we are not 'seeing' anything because we see with light and light just goes round atoms because its wavelength is too long for it to show anything. We could only see pictorial representations of how it interacts with electrons or, possibly, em waves of much shorter wavelength.
It's basically the electric field pattern around the atom which these pictures show.
 

Offline that mad man

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When we see an atom, what are we looking at?
« Reply #3 on: 13/04/2008 20:05:40 »
Its a computer generated image from a Scanning Tunnelling Microscope and as such we are not looking at atoms only a pictorial representation of what they may look like.

I think that the colours are false and have been added after for aesthetic enhancement otherwise it would all look monochrome. They do the same thing with Electron Microscopy, although with that they are in fact proper monochrome pictures and then falsely coloured in after by an artist.

Opps, sophicentaur just pipped me! :)
« Last Edit: 13/04/2008 20:07:12 by that mad man »
 

Offline chris

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When we see an atom, what are we looking at?
« Reply #4 on: 13/04/2008 21:41:58 »
So how does a scanning tunnelling microscope work?
 

Offline lightarrow

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When we see an atom, what are we looking at?
« Reply #5 on: 13/04/2008 22:05:35 »
So how does a scanning tunnelling microscope work?
For what I remember, a tiny metal needle, as sharp as a single tungsten atom on the tip, and connected to an electronic amplifier, "feels" a voltage variation when is near an atom's electronic cloud.
 

Offline lightarrow

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When we see an atom, what are we looking at?
« Reply #6 on: 13/04/2008 22:07:23 »
And, of course, we are not 'seeing' anything because we see with light and light just goes round atoms because its wavelength is too long for it to show anything. We could only see pictorial representations of how it interacts with electrons or, possibly, em waves of much shorter wavelength.
Don't understand why a single atom can't scatter visible light.
 

Offline graham.d

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When we see an atom, what are we looking at?
« Reply #7 on: 13/04/2008 23:52:12 »
It's to do with the wavelength and wave-particle duality. Because light behaves as an electromagnetic wave (or a probability wave if thinking of it as a particle), objects much smaller than the wavelength have little effect on the wave. It is only when the object's size gets to be similar to that of the wavelength that the wave gets significantly affected. Optical microscopes (with visible light) can't resolve images of objects below about 1000nm. They look rather fuzzy. This is because the wavelength of visible light is between 400nm and 700nm. You can still get photographic images by using ultra-violet light which extends down to about 10nm, but of course the eye can't see this directly. Semiconductor devices, with fine patterns, are still made using deep UV light and photoresists that react to the UV (in a similar way to how photographic film reacts chemically to visible light). To resolve images below these dimensions needs shorter wavelengths such as X-rays, but these are difficult to manage and translate into images. It is not easy to focus X-rays for example, although X-ray lithography is used in some cases. For even finer resolution electrons are used and they can be controlled with electric fields and magnets. They can also behave as a wave and have similar limitations but the effective wavelength is much shorter, allowing much finer image resolution.
 

lyner

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When we see an atom, what are we looking at?
« Reply #8 on: 14/04/2008 10:50:25 »
Don't understand why a single atom can't scatter visible light.
Molecular scattering of light definitely occurs in the atmosphere (the blue sky isn't just because of dust), according to my old Professor Ditchburn's book, 'Light'. But that doesn't mean that you can actually see an image. Scattering must involve some energy transfer. If an object is very small, all things being equal, then it will not interact with much of the incident radiation.
If the atom happens to be resonant at the incident frequency then it can absorb a photon and then re radiate it. The radiated wave will be omnidirectional (or at least cover a very wide angle).

With or without the resonant condition, you can look at things classically and consider how a simple electric dipole interacts with a radio wave. When the dipole is a half wavelength in length it is 'tuned' and will resonate strongly with the wave. A lot of power will be absorbed and re radiated (all around), giving strong scattering. (The thin wire actually behaves as if it has a large area as far as absorbing energy is concerned). As the dipole is shortened it is 'off resonance' and interacts less and less with the incident wave- producing less scattering. It presents less and less of a scattering cross section.
An atom will have a size which is a tiny fraction of optical wavelengths so it will scatter very little power.
Rayleigh scattering of light in the atmosphere demonstrates this effect; the amount of scattering is roughtly proportional to 1/λ at these wavelengths.
You can use extra components to tune (resonate) an antenna, artificially and you can increase its scattering cross section so that a very short dipole can interact with long wavelengths. Medium frequency receivers use a resonant antenna (ferrite rod) to extract significant energy from a radio broadcast although they may be a 1/5000 λ in size. This is a link to the quantum level interaction of atoms with waves.

Diffraction: In order to see an image, the object must produce a diffraction pattern which is recognisable. You have to 'see' a difference in brightness at different angles and, if the diffraction pattern is broader than the object then you just won't locate an image  in any particular place - it will be 'totally fuzzy' (to use a technical term). The image is dominated by the fuzziness of the edges and there is no significant central area which can be identified as 'the atom'.

Quote
It's to do with the wavelength and wave-particle duality.
Why make it more complicated than necessary, graham.d? If there's a reasonable classical explanation, why not use it?
« Last Edit: 14/04/2008 12:09:41 by sophiecentaur »
 

Offline graham.d

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When we see an atom, what are we looking at?
« Reply #9 on: 14/04/2008 13:54:18 »
Just to add further to this discussion, if somewhat off subject, light is "refracted" by even very low densities of atoms or charged particles. By this I mean that the velocity of light is slowed by passing through air (say) although not by very much and it is slowed differently at different wavelengths. One of the ways that it is possible to work out the distances of Pulsars is by using exactly this effect. It is known (as well as it can be) that pulsars emit bursts of electromagnetic radiation and that this radiation (Bremsstrahlung radiation) is emitted on all wavelengths simultaneously. Because there is a density of material within the interstellar "space" in our galaxy, there is a finite refractive index. This index varies with wavelength, which disperses the wavelengths in time so that different wavelengths arrive at different times (dispersion). By measuring the dispersion and by having a model of interstellar material within the galaxy, it is possible to get estimates of the distances of Pulsars, even though the desity of interstellar material is very low: perhaps of the order of 1 electron per cubic metre.

Not very relevent, but maybe interesting to some.
 

Offline Soul Surfer

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When we see an atom, what are we looking at?
« Reply #10 on: 14/04/2008 17:42:00 »
You can't "see" an atom stuck on the surface of other material because you would have to use light of such short wavelength that you would disloge the atom as it reflected the energy of the electromagnetic radiation you can howeer "feel it" a bit like rubbing your finger over a smoth polished surface with a grain of dust stuck on it.  that is effectively what the scanning tunnelling microscope does  it gently ffeels the electric fields at the surface of an atom.
 

Offline lightarrow

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When we see an atom, what are we looking at?
« Reply #11 on: 14/04/2008 18:10:05 »
Don't understand why a single atom can't scatter visible light.
Molecular scattering of light definitely occurs in the atmosphere (the blue sky isn't just because of dust), according to my old Professor Ditchburn's book, 'Light'. But that doesn't mean that you can actually see an image.
If an atom interacts with visible light, then the way to get an atom's image should however be (theoretically) possible.
Quote
Scattering must involve some energy transfer. If an object is very small, all things being equal, then it will not interact with much of the incident radiation.
Don't understand why the example of the blue sky that you made don't "catch" you. Even if the scattered light is a small percent of the incident one, you can amplify the signal as much as you want:
http://en.wikipedia.org/wiki/Image:Photomultiplier.svg
Quote

If the atom happens to be resonant at the incident frequency then it can absorb a photon and then re radiate it. The radiated wave will be omnidirectional (or at least cover a very wide angle).

With or without the resonant condition, you can look at things classically and consider how a simple electric dipole interacts with a radio wave. When the dipole is a half wavelength in length it is 'tuned' and will resonate strongly with the wave. A lot of power will be absorbed and re radiated (all around), giving strong scattering. (The thin wire actually behaves as if it has a large area as far as absorbing energy is concerned). As the dipole is shortened it is 'off resonance' and interacts less and less with the incident wave- producing less scattering. It presents less and less of a scattering cross section.
An atom will have a size which is a tiny fraction of optical wavelengths so it will scatter very little power.
Rayleigh scattering of light in the atmosphere demonstrates this effect; the amount of scattering is roughtly proportional to 1/λ at these wavelengths.
You can use extra components to tune (resonate) an antenna, artificially and you can increase its scattering cross section so that a very short dipole can interact with long wavelengths. Medium frequency receivers use a resonant antenna (ferrite rod) to extract significant energy from a radio broadcast although they may be a 1/5000 λ in size. This is a link to the quantum level interaction of atoms with waves.

Diffraction: In order to see an image, the object must produce a diffraction pattern which is recognisable. You have to 'see' a difference in brightness at different angles and, if the diffraction pattern is broader than the object then you just won't locate an image  in any particular place - it will be 'totally fuzzy' (to use a technical term). The image is dominated by the fuzziness of the edges and there is no significant central area which can be identified as 'the atom'.
Everything correct, but, again, I don't understand why should theoretically be impossible to have an atom's image.
 

Offline lightarrow

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When we see an atom, what are we looking at?
« Reply #12 on: 14/04/2008 18:37:04 »
It's to do with the wavelength and wave-particle duality. Because light behaves as an electromagnetic wave (or a probability wave if thinking of it as a particle), objects much smaller than the wavelength have little effect on the wave. It is only when the object's size gets to be similar to that of the wavelength that the wave gets significantly affected. Optical microscopes (with visible light) can't resolve images of objects below about 1000nm. They look rather fuzzy. This is because the wavelength of visible light is between 400nm and 700nm. You can still get photographic images by using ultra-violet light which extends down to about 10nm, but of course the eye can't see this directly. Semiconductor devices, with fine patterns, are still made using deep UV light and photoresists that react to the UV (in a similar way to how photographic film reacts chemically to visible light). To resolve images below these dimensions needs shorter wavelengths such as X-rays, but these are difficult to manage and translate into images. It is not easy to focus X-rays for example, although X-ray lithography is used in some cases. For even finer resolution electrons are used and they can be controlled with electric fields and magnets. They can also behave as a wave and have similar limitations but the effective wavelength is much shorter, allowing much finer image resolution.
If it could be possible to enlighten an atom (let's say as big as 10-10 m) and see it through a 1 mm lens using visible light (500 nm) at 10 atoms' distance, the resolution would be 5*10-13 m, that is you could see 500 parts of that atom.
« Last Edit: 14/04/2008 18:39:51 by lightarrow »
 

Offline techmind

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When we see an atom, what are we looking at?
« Reply #13 on: 14/04/2008 22:02:30 »
When IBM published their famous iconic image in 1990 showing IBM spelled out in atoms of xenon, the atoms resembled small grey footballs.

But what are we actually looking at when seeing those footballs, and how was that image produced?

...
So how does a scanning tunnelling microscope work?

As someone who wrote software to process images from an STM way back in ... 1988-1990, allow me to explain.  :)

In principle, you have a very fine probe "needle" tip (typically made of tungsten) and a conducting surface to observe. The needle tip is typically mounted on a 3-axis piezo transducer, so applying a high voltage to the piezo moves the tip around on a very fine scale. You may have a second, larger, piezo transducer and/or mechanical means to get coarse "tip-approach" control.

(A piezo-electric device changes its physical dimensions slightly in response to an applied electric field. The beepers in digital watches and the alarm sounder in consumer smoke-alarms are based on pizeo-transducers.)

To build an image, you normally apply a fixed potential between the tip and the surface, typically a volt or so, and measure the current flowing between the tip and the surface as the tip approaches. This current, the tunneling current, flows when the tip is close to the surface, but before it is in conventional contact. The current-flow is a very steep function of distance when the approach is sufficiently close.

To construct an "image" of the (electronic) profile of the sample-surface, you drive the 3-axis tip piezo to generate a raster-scanning motion sweeping across the surface, and using a feedback-loop, regulate the tip-surface distance (using the thrid piezo axis) to maintain a constant current between the tip and the surface.
The feedback signal is then linearly proportional to the "height" of the sample.

You then record the feedback signal as a function of the scanning raster position (using a computer), to build up a profile-map of the surface. Because of the steepness of the distance-current function, you can trace out the electron-clouds around individual atoms, and thus resolve the atomic configuration of the surface. This can then be represented on-screen as a contour-map or rendered as a 3D image complete with artificial "lighting" however desired.


Alternative STM designs may move the tip in only the "Z" direction while moving the surface in X-Y. "Other constructions will be apparent to those skilled in the art", etc etc.


Now although the tip-surface voltage is quite low (eg 1V, or a few 100mV) because the tip is so close to the surface, the electric field (Volts/distance) is extremely high. If you increase the voltage slightly you can exert sufficient force on the atoms on the surface (and the tip, which is why tungsten is used - very tough) to move them around. With a soft surface such as gold, you can blast a crater in it quite easily. IBM's xenon atoms probably bonded to the underlying substrate more weakly than the substrate bound to itself, so they could move the xenon without upsetting the substrate.

If IBM's publicity material image showed the xenon atoms in a different colour to the surface, then that would be entirely false colour based on the profile.
« Last Edit: 15/04/2008 22:14:59 by techmind »
 

Offline JP

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When we see an atom, what are we looking at?
« Reply #14 on: 14/04/2008 22:16:16 »
If it could be possible to enlighten an atom (let's say as big as 10-10 m) and see it through a 1 mm lens using visible light (500 nm) at 10 atoms' distance, the resolution would be 5*10-13 m, that is you could see 500 parts of that atom.

I suspect that you're using Fresnel diffraction theory (i.e. the resolution is ~λf/a, where f is the focal length, and a is the lens diameter).  However, you're far too close to the lens to use Fresnel diffraction, which requires that the angle that your lens subtends as you look at it from the focal point be small.  You'll need to do a full electromagnetic treatment of how the field interacts with the lens in order to calculate what you'll see at such a short distance.  My guess is that any image will be swamped out by evanescent field components (which quickly decay as you move further from the lens).  I don't know what kind of information you can extract from an image like that, but it certainly won't be a traditional "image" of the atom.

Now, if you want to use Fresnel diffraction theory, you need to be roughly paraxial.  For simplicity you could assume that your lens diameter and lens-to-image distance are approximately the same.  In that case, the smallest feature you can image is ~λ, the wavelength of the light. 
 

Offline lightarrow

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When we see an atom, what are we looking at?
« Reply #15 on: 14/04/2008 23:46:49 »
If it could be possible to enlighten an atom (let's say as big as 10-10 m) and see it through a 1 mm lens using visible light (500 nm) at 10 atoms' distance, the resolution would be 5*10-13 m, that is you could see 500 parts of that atom.

I suspect that you're using Fresnel diffraction theory (i.e. the resolution is ~λf/a, where f is the focal length, and a is the lens diameter).  However, you're far too close to the lens to use Fresnel diffraction, which requires that the angle that your lens subtends as you look at it from the focal point be small.  You'll need to do a full electromagnetic treatment of how the field interacts with the lens in order to calculate what you'll see at such a short distance.  My guess is that any image will be swamped out by evanescent field components (which quickly decay as you move further from the lens).  I don't know what kind of information you can extract from an image like that, but it certainly won't be a traditional "image" of the atom.

Now, if you want to use Fresnel diffraction theory, you need to be roughly paraxial.  For simplicity you could assume that your lens diameter and lens-to-image distance are approximately the same.  In that case, the smallest feature you can image is ~λ, the wavelength of the light. 
Yes, it's correct.
Then, what about negative index of refraction to overcome the diffraction limit?
http://physicsworld.com/cws/article/print/17398
<<one of the more provocative suggestions came from Pendry, who predicted that a slab of negative-index material could refocus the rays of a nearby source far better than the diffraction limit that is associated with all positive-index optics. In other words, it could lead to a "perfect lens">>

http://physicsworld.com/cws/article/news/22065
<<Physicists in the US have made an optical superlens from a thin layer of silver. The lens has a negative refractive index and can be used to image structures with a resolution that is about one sixth the wavelength of light -- thus overcoming the so-called diffraction limit (N Fang et al. 2005 Science 308 534). Xiang Zhang and colleagues at the University of California at Berkeley say that the lens could have many applications, such as imaging nano-scale objects with light>>.
« Last Edit: 15/04/2008 00:05:32 by lightarrow »
 

lyner

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When we see an atom, what are we looking at?
« Reply #16 on: 15/04/2008 12:49:06 »
Quote
Everything correct, but, again, I don't understand why should theoretically be impossible to have an atom's image.
(lightarrow)

'Theoretical' may not be 'practical' in this case. I think, in the end, it's a matter of signal to noise ratio (it often turns out that way). How may photons would you manage to get to hit your atom and then how would you detect them?
Things are much easier when you can use radio frequencies which don't involve quantum devices and you can sometimes signal process your way to an image. But, even then, wavelength is very relevant; why do they use microwaves for radar? Resolution, resolution ,resolution!
But the super resolution link looks interesting. . . .
« Last Edit: 15/04/2008 12:51:44 by sophiecentaur »
 

Offline chris

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When we see an atom, what are we looking at?
« Reply #17 on: 18/04/2008 09:29:00 »

As someone who wrote software to process images from an STM way back in ... 1988-1990, allow me to explain.  :)

In principle, you have a very fine probe "needle" tip (typically made of tungsten) and a conducting surface to observe. The needle tip is typically mounted on a 3-axis piezo transducer, so applying a high voltage to the piezo moves the tip around on a very fine scale. You may have a second, larger, piezo transducer and/or mechanical means to get coarse "tip-approach" control...

Thank you for the best explanation of scanning tunneling electron microscopy that I've ever read.

Chris
 

Offline syhprum

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When we see an atom, what are we looking at?
« Reply #18 on: 18/04/2008 22:04:03 »
The scanning tunneling microscope is a simple device in principle but is quite difficult to construct do to its sensitivity to vibration
 

Offline that mad man

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When we see an atom, what are we looking at?
« Reply #19 on: 18/04/2008 23:04:38 »
What amazes me about it is that you can get such a fine "needle" tip and move it with such precision.

How fine must the tip be to move just one atom only?
 

Offline techmind

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When we see an atom, what are we looking at?
« Reply #20 on: 20/04/2008 21:49:51 »
What amazes me about it is that you can get such a fine "needle" tip and move it with such precision.

How fine must the tip be to move just one atom only?

If you think about the end of a crudely-cut wire, it will still ultimately have one atom sticking forward of the rest. Owing to the quick drop-off of current, a less-than-ideal tip will very often work.
On the other hand if you've got more than one significant "tip" then you'll get a shadow/ghost image.
My dad got reasonable results just with a very fine piece of tungsten wire cut diagonally (with either fine wire cutters or a very sharp knife).

Indeed vibration can be a problem; very small and rigid mechanical construction helps considerably. Additional vibration-isolation ie heavy mass on top of an isolating suspension system is a good plan.


If you have access to IoP journals, have a look at
http://www.iop.org/EJ/abstract/0957-0233/1/9/009
"A versatile scanning tunnelling microscope for use in air"
WS Steer, B Hoffmann-Millack, CJ Roberts, WA Steer
Measurement Science and Technology (1990) 881-886
« Last Edit: 20/04/2008 21:55:21 by techmind »
 

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When we see an atom, what are we looking at?
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