materials chemistry

Pretty Pictures: Amorphous Growths of Europium, Thulium, and Hafnium

Showing off the prettier side of research is all the rage this week. From Beth Halford’s recent piece:http://pubs.acs.org/cen/science/85/8520sci2.html to psi’s latest blog post:http://coronene.blogspot.com/2007/05/sonogashira-party.html

In that spirit, here is a little collage with music of our more interesting/weird growths of some unsuccessful attempts in preparing uniform homogeneous non-radioactive targets.


Note 1: You can’t get any nerdier than adding a soundtrack to failed experiments. WinkMitch
By May 16, 2007 0 comments materials chemistry

Plasmonics Part II

Sorry for the long wait for this second post in the plasmonics series, but I got extra busy with school and the lab.

Anyways, before I start talking about what plasmons are and what they are good for, I want to thank “plasmonicsfocus” for pointing out a fact I failed to mention in my previous post.  The 10-6 transmission factor does not exactly represent the light passing through the tip.

See, classical diffraction theory predicts that the transmission of electromagnetic radiation through an aperture, of radius d, in a perfectly conducting plane obstacle should be given by,

T = (8/27)(pi)2(kd)4 + O[(kd)6]

So when the radius of the aperture is much smaller then the wavelength of the incoming wave, (d<<lambda) the transmission of energy through the aperture dies like (d*lambda)-4.

The catch here is that this analysis assumes the transmission is measured far from the aperture (far field diffraction). Far-field diffraction theory neglects those terms in the radiation field that die off faster than x-2 in the distance from the aperture.  So there is in fact, more energy that is interacting with the sample before the wave disperses to the far field.

For example, part of the transmitted energy emerges in an evanescent wave that travels along the reflecting interface between the media. Now it is possible to couple to this wave, despite the fact that its intensity diminishes exponentially with distance from the interface, and this allows one to create surface plasmons in thin metal films.

However I get ahead of myself.

Plasmons are plasma oscillations, or you could think of them as charge density waves.

Simply imagine a sphere comprised of many discrete, evenly-spaced positive charges, which we can approximate this as a positive charge distribution.  Now imagine a free electron cloud hovering just on top of the positive charge distribution, but not in contact.  The electrons are interacting with each other, and can be approximated as a negative charge distribution.  You have arrived at what is known as the Jellium model in metals.  These are plasmons in metal, and when the electrons move around, they are pulled back toward the positive charge distribution.  We assume they are not energetic enough to escape the E-field by the positively charged nuclei.  Thus they end up oscillating over/about the positive charge distribution.

In case you didn’t want to read all that, suffice it to say that plasmons in metal are basically vibrational modes of the electron gas density oscillating about the metallic ion cores.

Here is a picture to help you visualize it.

The oscillation frequency of the plasmon (wp), can be derived fairly easily after making the assumption that the E-field confining the electrons is harmonic.  Note how in the bottom right of the following picture, the frequency/waveguide (w/k = w*lambda) asymptotically approaches c.

Now surface plasmons describe the special case in which the charges are confined to the surface of the metal. In this case, the electric field is strongest in the plane of the metallic surface.  This is because the strength of the E-field dies off exponentially as you get further away from the charge distribution.

Plasmons confined to a plane do not radiate light, but when the local planar symmetry is disturbed, they can.  So unless you have a perfectly planar sample, and I really mean perfectly planar, the plasmons can radiate.  Surface plasmon-mediated emission from defects in metal surfaces has been observed, for example here.  These features have been described as “plasmon flashlights”; they are a localization of photon emission in a region generally experiencing non-radiative, collective oscillation of the surface electron density. Hence, It is possible to design structures that take advantage of this emission of surface plasmon radiation.

Lets take a look at a little more rigorous derivation of plasmon-light coupling:

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NOTE: The slides in this post were made by Jim Shuck, LBL. 

That is really the main point to take from this discussion.  Surface plasmons can oscillate at relatively low frequencies for relatively small wavelengths.  That’s why if you can turn light into plasmons (couple them), you can do sub-diffraction imaging.

Next time I’ll wrap all this up by talking about plasmonic nano-antennas.  Until then, if you’re interested you might want to look at these papers.

Toward Nanometer-Scale Optical Photolithography: Utilizing the Near-Field of Bowtie Optical Nanoantennas

Diffracted evanescent wave model for enhanced and suppressed optical transmission through subwavelength hole arrays 

By March 19, 2007 3 comments materials chemistry

Plasmonics

Have you ever felt more then a little dissatisfied with your SEM images?  Black and white just not doing it for you?  Wish you could see height differences at that small of a scale?  Just plain tired of babying that danged machine?  I know that more then once I’ve dreamed of using a bench top microscope to optically analyze our samples.  Some people have even tried (much to amusement of the rest of us) to jury-rig a set up to make this possible, but so far I’ve yet to here of any success (Mitch).

However, there is work being done on a method for being able to OPTICALLY analyze samples on the NANOMETER scale.  Yeah, it’s not bench top, but it has color, shadows, and did I say color?

And no, I’m not talking about NSOM, the method that is so lousy, inefficient, slow, and delicate that it seriously makes you question your commitment to science.

For those of you who don’t know what NSOM is, I’ll do a quick overview.  NSOM (Near Field Scanning Optical Microscopy), is a method of doing sub-diffraction imaging.  Remember that light is actually a self-propagating electromagnetic wave.  Like all waves, light diffracts when shoved through a tiny space (on the order of it’s wavelength).  Back in your highschool or lower division physics classes, you probably messed around with single slit diffraction and saw neat patterns like this.

Well, to shine very coherent light “spots” on your samples so that you can see them, you need a very very small aperture.  However when you go to use one that has a diameter close to or less then the wavelength of the light you send through, you see the light diffract.

So if you are trying to look at a sample and want to resolve details with <1000nm with visible light, your in trouble.  NSOM “gets around” this though, by sticking the NSOM tip very very close to the sample.  As in around 25-50nm away from it.  Why does this work?  When you get the tip that close to the sample, it is acting like a light source because you are inside the first diffraction maximum.

The setup kind of looks like this:

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Jim Schuck, Molecular Foundry, LBNL

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Now that all seems fine, but inherently you’re hitting another problem here.  The tip is so much smaller then your wavelength that it greatly prohibits the photons from getting through.  Of course some will, but now it is a matter of statistics.  The math has been cranked and the transfer efficiency of NSOM is only about 10-6!!  That means 1 in 1 billion photons sent at the tip gets through!  LOUSY

So now we get to the brighter solution.  heh.

Or actually, we won’t just yet.  This post is starting to get long, and I am starting to get tired.  I think I’ll do this in a 3 post manner.

1.) NSOM and why is bites.  2.) What the heck are Plasmons?  3.) Plasmon-light coupling and Nano-antennas.

Stay tuned…

-Maz

By March 7, 2007 0 comments materials chemistry

New guy (me) and some Pretty Pictures

Hello readers of Chemical Forums Chemistry Blog!  My name is Maz and starting today, I’ll be blogging on this site along with Mitch and another fellow named ‘movies’.

Oh and because I always get asked this question by lots of people; yes my real name is Maz.  No it isn’t the most creative user name, but I don’t think there are many other chemical bloggers name Maz out there.  If there are…well I am more special then them.

I am sure there are some of you who also frequent the forums and are wondering who the heck I am.  While my profile says I’ve posted 102 times, this is actually my first post on the site.  I am here because starting back in December, I started working with Mitch up at LBL, Berkeley’s science lab.  After a while he showed me more about this website and when I found out he was looking for a co-blogger, I thought it could be a fun distraction from physics, chemistry, and our research.

Speaking of our research, a while back Mitch posted some pictures of the spin coater we designed (ctrl+f: pimp my spin coater), and before that he had posted some SEM images of some Eu2O3 films he had made.

Well, I thought I’d offer a little before and after here, so you can see that spin coating is indeed a viable method for radioactive target development.   Check these babies out:

Before:

After:

Note the scale difference.  The first picture is at 1mm across the bar, the second is at 20um! WAY Better.

Before:

After:


Granted, the old pictures are more fun to look at, but they aren’t at all what we need.  We are trying to make a crack-free homogeneous thin film.  The new pictures show that we are getting very close.

But that isn’t to say that we don’t have the occasional contaminant.  Check this out:

Isn’t it just beautiful? I am pretty sure that came from the potassium we failed to filter out completely in our older process.  Since then, we have eliminated the addition of base to our polymer solution.

These next couple of images are simply an optical zoom from a Profilometer are of layers with a much higher concentration of the metal (by ~x90).  We know they aren’t even close to the crack-free homogeneous thin film we are looking for, but they are so darn pretty.  I figure that the higher conc. of metal is enough that we are seeing some funky polycrystalline structure arising instead of the epitaxial growth we want.  I wonder, if I greatly slowed down the growth (lower the temp, b/c atm we are blasting it) I would see a more homogeneous growth.

Moving right to left:  The multi-colored stuff is the HfO2 followed by the Si wafer (super-reflective stuff) followed by the platform.

I’m not yet sure what happened here.  Perhaps the light (source above) hit the cracks, which are a few micrometers wide, diffracted, reflected, and came out interfering to give a neat green color?  Whatever the case, it’s awesome looking.

See that is the best part about science.  We thought we were just about done with spin coating and the first 1/3 or so of this project.  I had figured that I knew just about all the parts of spin coating that were relevant to our project.  Sure enough, something entirely new and unexpected came up.

And now for something completely different:
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Need a spin coater?  For just $500 you will get your very own thin-film processing, spin coating device!  Complete we a variable speed knob, stand, wafer-attaching-mechanism, and 3 cool colors in L.E.D’s!!.  For a limited time only, we’ll even include an instructional video with Mitch and I showing you how to mount your wafers and add your solutions!  Don’t waste thousands of dollars on those old, dull, monochromatic coaters!  Spin with STYLE! Call 1-800-PIMP-COAT now!
Yes, our number is based in London.  I couldn’t think of a cool way to use only seven letters.

1.)  Next time I’ll try and remember to post a little more about myself.
2.)  Wish me luck (at least mentally) for my quantum midterm tomorrow.

Until next time I leave you with this article that some may find interesting, but I found funny. (I am a physics and chem double major, so maybe that is why)

Claiming Einstein for Chemistry

-Maz

By March 4, 2007 0 comments materials chemistry